We decide in this ease whether Jefferson County, Alabama, may impose on federal judges holding office under Article III of the Constitution1 a tax for the privilege of engaging in their occupation within the county. We hold that such a tax violates the Supremacy Clause of the Constitution.2
I. FACTS AND PROCEDURAL HISTORY
Jefferson County, Alabama, sued William M. Acker, Jr., and U.W. demon, United States District Judges for the Northern District of Alabama, to recover delinquent county taxes due under Jefferson County Ordinance No. 1120. Ordinance No. 1120 imposes a license or privilege tax (the “privilege tax”) on persons not otherwise required to pay any license or privilege tax to the State of Alabama or Jefferson County. The ordinance provides:
It shall be unlawful for any person to engage in or follow any vocation, occupation, calling or profession ... within the County on or after the 1st day of January, 1988, without paying license fees to the County for the privilege of engaging in or following such vocation, occupation, calling or profession, which license fees shall be measured by one-half percent ($.%) of the gross receipts of each such person.
Jefferson County, Ala., Ordinance No. 1120, § 2 (Sept. 29,1987).
*1564The ordinance defines “vocation, occupation, calling and profession” to include the holding of any kind of office, by election or appointment, by any federal, state, county, or city officer or employee where the officer’s or employee’s services are rendered within Jefferson County. Id. § 1(C).3 It is undisputed that the ordinance facially applies to federal judges. Non-residents of Jefferson County performing work in Jefferson County must pay the privilege tax. See id. § 1(B). The ordinance defines “gross receipts,” by which it measures the privilege tax, as the total gross amount of all salaries, wages, or other monetary payments of any kind which a person receives or is entitled to receive for work or services. Id. § 1(F).4 If compensation is earned from work both inside and outside Jefferson County, the privilege tax is based on the proportion of work performed within Jefferson County. Id. § 3. The computation of the percentage of work done within Jefferson County must be supported by oath. Id.
The ordinance requires employers to withhold privilege taxes, to file returns with the Director of Revenue, and to keep and maintain certain records for five years. Id. § 4. The Administrative Office of the United States Courts has never withheld Jefferson County privilege taxes from the salary of any federal judge or court employee. Under the ordinance, an employer’s failure to withhold the privilege tax does not relieve employees from the obligation to pay. Id. An employee whose employer has failed to comply with the ordinance must file a return and pay the privilege tax. Id. § 5.
The ordinance grants certain investigative powers to the Jefferson County Director of Revenue. These include the power to examine the books, records, and papers of any employer or licensee to determine the accuracy of any return or to determine the amount of privilege taxes due if no return was filed, as well as the power to examine any person under oath concerning any gross receipts which were or should have been shown in a return. Id. § 7. The Director of Revenue also may promulgate regulations for the administration and enforcement of the ordinance. Id. § 8.
The ordinance imposes interest and penalties for the failure to pay privilege taxes and the failure to withhold privilege taxes. Id. § 10(A). In addition, the ordinance alludes to other punishment for failing to comply with its requirements:
Any person or employee who shall fail, neglect or refuse to pay a license fee ... or any employer who shall fail to withhold said license fees, or to pay over to County such license fees ..., or any person required to file a return ... who shall fail, neglect or refuse to file such return, or any person or employer who shall refuse to permit the Director of Revenue or any *1565agent or employee designated by him ... to examine his books, records and papers for any purpose authorized by this Ordinance ... shall upon conviction be subject to punishment within the limits of and as provided by law for each offense. Such punishment shall be in addition to the penalties imposed under subsection (A) of this section.
Id. § 10(B). Alabama law provides that each violation5 of a city or town ordinance requiring the payment of privilege taxes is punishable by a fine, as prescribed by the ordinance, of up to $500, by up to six months imprisonment, or by both. Alabama Code § 11-51-93 (1989). Alabama law does not appear to provide criminal sanctions for violating county ordinances requiring the payment of privilege taxes.
At least three other local governments in Alabama have ordinances requiring the payment of license or privilege taxes. The Cities of Gadsden and Birmingham, in the Northern District of Alabama,6 and Auburn, in the Middle District of Alabama, have ordinances almost identical to Jefferson County’s, though their ordinances tax gross receipts at a higher rate and, because they are city ordinances, are backed by criminal penalties under Alabama law. See id. Counsel for Jefferson County told us at oral argument that Jefferson County simply copied Birmingham’s ordinance when enacting Ordinance No. 1120.
Judge Acker and Judge Clemon maintain their principal offices in the Hugo Black Federal Courthouse in Birmingham, Alabama, which lies within Jefferson County. They routinely perform some but not all of their duties outside of Jefferson County. Judges Acker and Clemon have refused to pay the privilege tax imposed by the ordinance. Before the district court’s opinion in this case, all other active judges of the Northern District of Alabama paid the privilege tax on differing percentages of their judicial salaries without supporting those percentages by an oath or any formal accounting procedure. In addition, all state judges with offices in Jefferson County have paid the privilege tax based on portions of their salaries.
Jefferson County sued Judge Acker and Judge Clemon in state court to recover delinquent privilege taxes due under the ordinance. Each judge removed his case to federal court, where the cases were consolidated. The parties stipulated to the facts and submitted cross-motions for summary judgment.
The district court7 held that, under the intergovernmental tax immunity doctrine, the ordinance is unconstitutional as applied to Judge Acker and Judge Clemon. The court concluded that the legal incidence of the privilege tax falls on the federal judicial function. Jefferson County v. Acker, 850 F.Supp. 1536, 1543 (N.D.Ala.1994). According to the court, the privilege tax, “by express intention and in real effect, is a franchise tax imposed upon the federal judicial operations and is unconstitutional as a direct tax upon an officer and instrumentality of the United States, that is, upon the sovereign itself.” Id. at 1545-46.
The district court also held that applying the ordinance to Judges Acker and Clemon violates the Compensation Clause of Article III.8 Id. at 1547. The privilege tax diminishes a judge’s compensation, rather than taxing his salary, the court held, because its incidence “is upon the performance of judicial functions by a judicial officer, antecedent to the point that the salary therefor having been paid by the government becomes the property of the individual citizen of Alabama.” Id. at 1547-48. Jefferson County *1566appealed.9
A panel of this court reversed, holding that the ordinance may be applied to Article III judges without violating the intergovernmental tax immunity doctrine or the Compensation Clause. Jefferson County v. Acker, 61 F.3d 848 (11th Cir.1995). Chief Judge Tjoflat dissented. The panel majority disagreed with the district court’s conclusion that the ordinance taxes the federal judicial function. The panel majority determined that “the practical effect of [the ordinance] is to tax the income that federal judges derive from the performance of their judicial functions,” not “to impose a license tax as a precondition to the performance of those functions.” Id. at 855. And the panel majority determined that federal judges are federal officers rather than arms of the federal government. Id. at 853. Therefore, the panel held, the ordinance does not directly tax the operations of the federal government in violation of the intergovernmental tax immunity doctrine. Id. at 856.
Also based on its determination that the practical effect of the privilege tax is that of an income tax, the panel majority held that the Compensation Clause does not bar applying the ordinance to federal judges. Id. According to the panel majority, “[i]t is well established that the Compensation Clause does not forbid ... levying an income tax on federal judges.” Id. (citing O’Malley v. Woodrough, 307 U.S. 277, 282, 59 S.Ct. 838, 840, 83 L.Ed. 1289 (1939)).
Judges Acker and Clemon filed a suggestion for rehearing en banc. Recognizing this case to involve legal questions and principles of exceptional importance, we granted rehearing en banc to determine whether the ordinance constitutionally may be applied to Article III judges.
II. ISSUES ON APPEAL
Two issues have been raised on appeal: (1) whether the tax imposed by Ordinance No. 1120 constitutes an unconstitutional diminution in the compensation of Article III judges; and (2) whether the tax imposed by Ordinance No. 1120 violates the Supremacy Clause as an intergovernmental tax. Because we hold that the Supremacy Clause bars the application of the ordinance to federal judges, we do not address whether the ordinance unconstitutionally diminishes federal judges’ compensation.
III. CONTENTIONS OF THE PARTIES
Jefferson County contends that the district court erred in holding that the intergovernmental tax immunity doctrine prohibits imposing the privilege tax on federal judges. Jefferson County argues that the Public Salary Act and the Buck Act waived the tax immunity of federal officers, including federal judges, with respect to all taxes except discriminatory taxes. Because the privilege tax is not discriminatory, the County argues, it constitutionally may be applied to federal judges.
The County further contends that, even if Congress’s waiver of federal tax immunity does not apply to the privilege tax, the intergovernmental tax immunity doctrine bars only those state taxes levied directly on the federal government itself.10 The privilege tax, the County argues, is not levied directly on the federal government. Rather, it is imposed on individuals, who are employees of the federal government as opposed to its agencies or instrumentalities. The County argues that Judges Acker and Clemon have conceded their tax immunity argument by admitting that they are subject to the Alabama state income tax: if they were instru-mentalities of the federal government, tax immunity would shield them not only from *1567the privilege tax but also from state income taxes.
Judges Acker and Clemon contend that Congress has not waived their federal tax immunity from the privilege tax. They argue that the privilege tax violates the intergovernmental tax immunity doctrine because the legal incidence of the privilege tax is not on the individual judge but on the performance of the federal judicial function. The judges contend that a federal judge is the federal court when performing judicial duties. The judges contend that state law is determinative of the legal incidence of the privilege tax. When state law demonstrates that a tax is levied on a federal function, they argue, the practical effect of the tax need not be considered. Judges Acker and Clemon also argue that the ordinance’s onerous timekeeping and return requirements burden the federal judicial function.
IV. DISCUSSION
We are presented with an issue of first impression. The parties have not cited, and we have not found, any federal ease addressing whether the intergovernmental tax immunity doctrine prohibits a state or local government from imposing a privilege tax on Article III judges.
We begin our analysis with an examination of the contours of the intergovernmental tax immunity doctrine, mindful that the nature of the tax and the identity of the taxpayer here differ significantly from the taxes and taxpayers at issue in previous intergovernmental tax immunity cases. Then we apply the doctrine to the judges’ challenge to the Jefferson County privilege tax. Finally, we determine whether the Public Salary Act and the Buck Act have altered the intergovernmental tax immunity doctrine’s limits on state and local taxation so as to permit the imposition of the privilege tax on federal judges.
A. The Intergovernmental Tax Immunity Doctrine
The purpose of the intergovernmental tax immunity doctrine is to forestall “clashing sovereignty.” United States v. New Mexico, 455 U.S. 720, 735, 102 S.Ct. 1373, 1383, 71 L.Ed.2d 580 (1982) (quoting McCulloch v. Maryland, 4 Wheat 316, 430, 4 L.Ed. 579 (1819)). Born of Chief Justice Marshall’s opinion in McCulloch v. Maryland, and aphoristically expressed in Marshall’s famous dictum “the power to tax involves the power to destroy,” the intergovernmental tax immunity doctrine seeks to reconcile states’ sovereign taxing authority with the Supremacy Clause’s protection of federal operations from state interference. See generally New Mexico, 455 U.S. at 730-36, 102 S.Ct. at 1380-1383; Paul J. Hartman, Federal Limitations on State and Local Taxation §§ 6:1-6:15 (1981). The Supreme Court’s attempt to fashion a doctrine accommodating these competing constitutional imperatives “has been marked from the beginning by inconsistent decisions and increasingly delicate distinctions.” New Mexico, 455 U.S. at 730, 102 S.Ct. at 1380-81.
For over a century, the Supreme Court treated Marshall’s famous dictum as a constitutional mandate, Graves v. New York ex rel. O’Keefe, 306 U.S. 466, 489, 59 S.Ct. 595, 602, 83 L.Ed. 927 (1939). (Frankfurter, J., concurring), finding in ease after case that nondiseriminatory state taxes potentially affecting the federal government — even taxes imposed on private parties dealing with the government — threatened to disrupt federal operations. The Court thus struck down, for example, state income taxes on federal employees, Dobbins v. Commissioners, of Erie County, 41 U.S. (16 Pet.) 435, 10 L.Ed. 1022 (1842), and state sales taxes on private companies’ sales to the federal government, Panhandle Oil Co. v. Mississippi ex rel. Knox, 277 U.S. 218, 48 S.Ct. 451, 72 L.Ed. 857 (1928). The theory was that such taxes might increase the cost to the federal government of performing its functions. United States v. County of Fresno, 429 U.S. 452, 460, 97 S.Ct. 699, 703, 50 L.Ed.2d 683 (1977).
The theory that a nondiseriminatory tax unconstitutionally interferes with federal functions simply because it imposes an economic burden on the federal government was abandoned in James v. Dravo Contracting, 302 U.S. 134, 58 S.Ct. 208, 82 L.Ed. 155 (1937). There, the Court assumed that a *1568state gross receipts tax levied on a federal contractor increased the cost to the government of the contractor’s services, but held that the tax nevertheless did not interfere in any substantial way with the performance of federal functions. Id. at 160, 58 S.Ct. at 221. Dravo signalled the beginning of the end of constitutional tax immunity for private parties dealing with the federal government. Thus, two years later the Court overruled Dobbins, which had immunized federal employees from state income taxes, declaring that any economic burden on the government from an income tax on a government employee is “but the normal incident of the organization within the same territory of two governments, each possessing the taxing power,” and a burden “which the Constitution presupposes.” Graves v. New York ex rel. O’Keefe, 306 U.S. 466, 487, 59 S.Ct. 595, 601, 83 L.Ed. 927 (1939) (“O’Keefe ”).
The O’Keefe Court focused its analysis on whether an income tax on a federal employee obstructs or interferes with the performance of federal functions. Id. at 477, 481, 484, 59 S.Ct. at 597, 599, 600. Earlier cases granting immunity from income taxes, the Court said, failed to consider whether such taxes interfered with government functions; they just assumed that the immunity of the government and its instrumentalities extended to employees of those entities. Id. at 481, 59 S.Ct. at 599. But “[t]he theory ... that a tax on income is legally or economically a tax on its source [was] no longer tenable” after Dravo. Id. at 480, 59 S.Ct. at 598. Thus not willing to assume any burden on government functions, id. at 486, 59 S.Ct. at 601, the court examined whether an income tax indeed interfered with government functions. The Court found no burden on federal functions other than the economic burden that may be passed on to the government in the form of higher labor costs. Id. at 481, 59 S.Ct. at 598. Concluding that such a burden does not amount to an interference with the performance of federal functions, the Court upheld the imposition of state income taxes on federal employees. Id. at 487, 59 S.Ct. at 601.
Later cases similarly recognized that the economic burden on the federal government of nondiscriminatory state taxes imposed on those dealing with the federal government generally does not threaten to impede the performance of federal functions. E.g., South Carolina v. Baker, 485 U.S. 505, 521, 108 S.Ct. 1355, 1366, 99 L.Ed.2d 592 (1988) (noting that tax’s entire financial burden may fall on government without rendering tax unconstitutional); New Mexico, 455 U.S. at 734, 102 S.Ct. at 1382 (noting that no immunity arises from federal government shouldering tax’s entire economic burden); County of Fresno, 429 U.S. at 462, 97 S.Ct. at 704-705 (noting that economic burden on federal function does not render tax unconstitutional). With this recognition, the intergovernmental tax immunity doctrine has become somewhat more attuned to the practical realities of our federal system. But the test for determining whether a nondiscriminatory tax interferes with the federal government’s functions remains highly formalistic.
Current intergovernmental tax immunity doctrine asks whether the “legal incidence,” as opposed to the economic burden, of the tax falls directly on the federal government or its instrumentality. See New Mexico, 455 U.S. at 735, 102 S.Ct. at 1383; County of Fresno, 429 U.S. at 464, 97 S.Ct. at 705. A nondiscriminatory state or local tax is unconstitutional only “when the levy falls on the United States itself, or on an agency or instrumentality so closely connected to the Government that the two cannot realistically be viewed as separate entities, at least insofar as the activity being taxed is concerned.” New Mexico, 455 U.S. at 735, 102 S.Ct. at 1383. To be an instrumentality of the government, a taxed entity must be “so intimately connected with the exercise of a power or the performance of a duty by the Government that taxation of it would be a direct interference with the functions of government itself.” Id. (citations and internal quotation marks omitted).
The “legal incidence” test has significantly constricted federal intergovernmental tax immunity. Indeed, the Supreme Court has characterized the current doctrine’s prohibition against taxes legally incident on the federal government or its instrumentalities as of “essentially symbolic importance, as the *1569visible ‘consequence of that [federal] supremacy which the constitution has declared.’” New Mexico, 455 U.S. at 735, 102 S.Ct. at 1383 (quoting McCulloch v. Maryland, 4 Wheat, at 436). Relegation of the doctrine to largely symbolic importance is not surprising in light of the recognition that the economic burden of nondiscriminatory state taxes does not threaten the government’s operations. After all, by its very essence, a tax imposes an economic burden. If the Constitution presupposes such an economic burden, then few taxes will violate the intergovernmental tax immunity doctrine.
We do not mean to gainsay the intergovernmental tax immunity doctrine’s importance in our federal system. Though it has been narrowed and beset by formalism, the doctrine has continuing vitality. Our point is that the reason for the doctrine’s contraction must be appreciated to understand the scope of the doctrine’s continuing vitality. The doctrine’s contraction stemmed not from a weakening of the principle that, under the Supremacy Clause, states may not burden or interfere with federal operations, but from the recognition that nondiscriminatory taxes levied on private parties generally do not impede federal operations. The intergovernmental tax immunity doctrine still prohibits any state or local tax that burdens or interferes with federal operations.
Mindful of the underlying purpose of intergovernmental tax immunity, the doctrine’s history, and the “actual workings of our federalism,” O’Keefe, 306 U.S. at 490, 59 S.Ct. at 603 (Frankfurter, J., concurring), we turn to whether the Jefferson County privilege tax constitutionally may be levied on Judges Acker and demon.
B. The Federal Judges’ Challenge to the Privilege Tax
Judge Acker and Judge demon’s challenge to the privilege tax differs substantially from most intergovernmental tax immunity challenges. As far as we can tell, Judges Acker and demon are the first federal judges to challenge a state or local tax on intergovernmental tax immunity grounds. Moreover, because the privilege tax differs from most taxes, their objection to the privilege tax is novel. They do not allege that the privilege tax interferes with federal functions by imposing an economic burden on the federal government. The district court found that the privilege tax imposes no economic burden on the federal government itself; it is paid by individual federal judges out of then-own pockets. Judges Acker and demon do not question this conclusion and, thus, do not make the economic-burden argument that now has been thoroughly repudiated by the intergovernmental tax immunity doctrine.11
The burden of which Judges Acker and demon complain is the ordinance’s requirement that they remit privilege taxes for the privilege of lawfully performing federal judicial duties in Jefferson County. Though they object to paying a tax, they do so not for the economic reasons generally associated with objections to taxes but because the tax purports to be a precondition to the lawful performance of their federal judicial duties.
Jefferson County contends that the privilege tax does not regulate, control, or license a federal judge’s performance of his duties any more than a state income tax. If Jefferson County is correct that, despite being labelled a “license fee,” the privilege tax amounts to an income tax, then it constitutionally may be applied to Judges Acker and demon under O’Keefe. Thus, before attempting to ascertain the “legal incidence” of the privilege tax under the intergovernmental tax immunity doctrine, we examine the substantive nature of the privilege tax to determine whether it merely taxes the receipt of income.
1. Whether the Privilege Tax Is In Substance An Income Tax
To determine the nature and effect of the privilege tax, “we must look through form and behind labels to substance.” City of Detroit v. Murray Corp. of America, 355 U.S. 489, 492, 78 S.Ct. 458, 460, 2 L.Ed.2d 441 (1958). We are the ultimate arbiters of *1570the substance of the privilege tax. But state law defines the attributes comprising the substance of the privilege tax..
The Alabama Supreme Court has described the operational effect of a City of Auburn ordinance identical to the Jefferson County ordinance in all relevant respects. McPheeter v. City of Auburn, 288 Ala. 286, 259 So.2d 833 (1972). Rejecting the argument that tbe Auburn ordinance imposed an income tax not authorized by the state constitution, Alabama’s highest court explained that
[t]he tax is occasioned when the taxpayer performs services within the Auburn city limits, and not when the taxpayer receives income. Therefore, the ordinance taxes the privilege of working and the engagement of rendering services within the City of Auburn, and it only measures the tax due by the amount of the taxpayers’ gross receipts which result from such privilege. ... It is evident that the tax is not even measured by a person’s income, but only by his salary or wages earned. So in no sense can the Auburn tax be considered an income tax.
Id. at 837.
Concerned with substance, not labels, we pay no heed to the state court’s conclusion that the privilege tax is not an “income tax” under state law. In analyzing the privilege tax’s natural effect, however, we accord great weight to the state court’s determination of how the tax operates; if the state court’s determination is a reasonable interpretation of the ordinance, we deem it conclusive. See Gurley v. Rhoden, 421 U.S. 200, 208, 95 S.Ct. 1605, 1610, 44 L.Ed.2d 110 (1975) (deferring to state court’s reasonable determination of operating incidence of excise tax).
The Alabama Supreme Court’s determination of the operation of the Auburn ordinance is a reasonable interpretation of how the identical Jefferson County ordinance operates. Our examination of the Jefferson County ordinance, within the context of Alabama law, reveals that the privilege tax is a tax on the performance of work in Jefferson County. In substance, the privilege tax does not tax the receipt of income.
The privilege tax differs fundamentally from an income tax. The ordinance purports to make it unlawful to engage in one’s occupation in Jefferson County without paying the privilege tax. Ordinance No. 1120, § 2. This provision indicates that, instead of taxing the receipt of income, the privilege tax attaches to the performance of work in Jefferson County.
Other provisions of . the ordinance further demonstrate that the privilege tax does not merely tax the receipt of income. The privilege tax is levied not only on income received but also on income that one is entitled to receive, id. § 1(F), indicating that the ordinance is concerned with ensuring that work is taxed regardless of whether income from the work actually is received. Moreover, persons engaged in occupations or businesses for which they are required to pay state or other Jefferson County license fees are exempted from paying the privilege tax under Ordinance No. 1120. Id. § 1(B). We do not understand why, if the ordinance is an income tax, it exempts from its requirements persons paying license fees to Jefferson County or to the State of Alabama, license fees that are totally unrelated to income.12 This exemption makes sense only if the ordinance aims to ensure that a license fee is paid to some unit of government for all work performed in Jefferson County.
We hold that the Jefferson County privilege tax is not, in substance, a tax on income. Though the privilege tax is measured by .income, at least roughly, its other attributes remove it from any reasonable conception of an income tax. Therefore, this case is not .controlled by O’Keefe’s holding that income taxes do not interfere with federal functions in violation of the intergovernmental tax immunity doctrine.
*15712. The Legal Incidence of the Privilege Tax
Our determination that the privilege tax does not tax the receipt of income is only the beginning of our inquiry. Regardless of what “type” of tax the privilege tax is, the intergovernmental tax immunity doctrine bars its imposition on Judges Acker and Clemon only if its legal incidence falls directly on the federal government or its instrumentality. New Mexico, 455 U.S. at 735, 102 S.Ct. at 1383. Judges Acker and Clemon urge that the privilege tax falls on the federal judicial function, as the district court held. Jefferson County contends that the privilege tax is imposed on individuals, not on the federal government or the federal judicial function.
Identifying the legal incidence of the privilege tax is a question of federal law. Kern-Limerick, Inc. v. Scurlock, 347 U.S. 110, 121, 74 S.Ct. 403, 410, 98 L.Ed. 546 (1954). However, as with our determination of the nature of the privilege tax, determining the privilege tax’s legal incidence requires us to identify the substantive characteristics of the privilege tax under state law. City of Detroit, 355 U.S. at 493, 78 S.Ct. at 460-61. Then, we must evaluate the substance of the privilege tax under the federal standards for identifying a tax’s legal incidence. Kern-Limerick, 347 U.S. at 121, 74 S.Ct. at 410.
We hold that the legal incidence of the tax falls on the federal judge. As the Supreme Court seems to apply the legal incidence test, the legal incidence of a tax falls on the entity that the taxing statute identifies as the taxpayer and contemplates paying the tax. See United States v. State Tax Commission of Mississippi 421 U.S. 599, 607-610, 95 S.Ct. 1872, 1877-79, 44 L.Ed.2d 404 (1975); Gurley, 421 U.S. at 203-212, 95 S.Ct. at 1608-12; Kern-Limerick, 347 U.S. at 113-123, 74 S.Ct. at 406-411. The ordinance identifies the person engaging in work in Jefferson County as the taxpayer and contemplates that he or she will pay the tax.13 Ordinance No. 1120, §§ 2, 4, 5. Thus, the legal incidence of the privilege tax falls on Judge Acker and Judge Clemon.
3. Whether Federal Judges Are Federal Instrumentalities
? must determine, then, whether Judges Acker and Clemon may be considered the federal government or its instrumentalities. The district court concluded that federal judges are federal- instrumentalities. Judges Acker and Clemon argue that a federal judge is the federal court when performing judicial duties. Jefferson County argues that Judges Acker and Clemon are individuals and employees of the federal government, not its instrumentalities. According to the County) Judges Acker and Clemon cannot be instrumentalities of the government because, if they were, then they would be immune from state income taxes as well.
Judges Acker and Clemon may be instru-mentalities of the federal government with respect to the taxation of one activity but not another. See New Mexico, 455 U.S. at 740-743, 102 S.Ct. at 1386-87 (suggesting that an entity may be a federal instrumentality when one activity is taxed even if it is not an instrumentality when another activity is taxed). The Supreme Court’s description of what constitutes a federal instrumentality suggests that the activity being taxed may determine whether the taxpayer is a federal instrumentality. Tó be an instrumentality, an entity must be “so closely connected to the Government that the two cannot realistically be viewed as separate entities, at least insofar as the activity being taxed is concerned,” or “so intimately connected with the exercise of a power or the performance of a duty by the Government that taxation of it would be a direct interference with the functions of government itself.” Id. at 735, 102 S.Ct. at 1383 (citations and internal quotation marks omitted) (emphasis added).
We accept that a federal judge is not an instrumentality of the federal government when the activity being taxed is the judge’s receipt of income. A judge may be no more intimately connected with the federal government when receiving income than the federal *1572employee in O’Keefe. The taxation of a federal judge’s income may interfere with the functions of government no more than the taxation of any other federal employee’s income. But taxing a federal judge in the performance of his or her judicial duties is fundamentally different from taxing his or her income.
When performing federal judicial duties, a federal judge performs “the functions of government itself,” New Mexico, 455 U.S. at 735, 102 S.Ct. at 1383, and cannot realistically be viewed as a separate entity from the federal court. The judge is “so intimately connected with the exercise of [federal judicial] power or the performance of a [federal judicial] duty ... that taxation of [him] would be a direct interference with the functions of government itself.” Id. Thus, we hold that a federal judge is a federal instrumentality when the taxed activity is the judge’s performance of judicial duties.
We conclude, then, that the intergovernmental tax immunity doctrine bars the imposition of the Jefferson County privilege tax on Judges Acker and Clemon. The privilege tax taxes the activity of working in Jefferson County. As applied to Judges Acker and Clemon, the privilege tax taxes the performance of federal judicial duties in Jefferson County. When performing their judicial duties, Judges Acker and Clemon must be considered instrumentalities of the federal government. The imposition of the privilege tax on Judges Acker and Clemon, therefore, amounts to a direct tax on federal instrumen-talities in violation of the intergovernmental tax immunity doctrine.
Our conclusion that the Constitution bars levying the privilege tax on Judges Acker and Clemon follows not only from a formal application of the intergovernmental tax immunity doctrine but also from adherence to the doctrine’s overarching purpose. The imposition of the privilege tax on federal judges is apt to lead to the clashing sovereignty that the Supremacy Clause seeks to avoid. By its very terms and in practical effect, Ordinance No. 1120 may be applied to federal judges only at the risk of interfering with the operation of the federal judiciary.
According to its plain language, the ordinance makes it unlawful for a federal judge to perform his or her duties in Jefferson County without paying the privilege tax. The County argues that Alabama counties have no power to prosecute anyone criminally for failure to pay the privilege tax.14 While Alabama counties currently lack the power to impose criminal sanctions for failure to pay the privilege tax, the comfort that this omission provides may be short-lived; the Alabama legislature could of course provide a criminal penalty provision applicable to counties like the provision applicable to cities and towns.15
Regardless of whether a county possesses the power under Alabama law to make unlicensed work a crime, a federal judge in Jefferson County who for some reason fails to pay the privilege tax is deemed by Jefferson County to act unlawfully when he performs his judicial duties. We have no doubt that, under the Supremacy Clause, Jefferson County could not enjoin or otherwise prevent a federal judge from performing federal duties. But we believe that the Supremacy Clause protects the federal judiciary not only from outright obstruction but also from a requirement that a federal judge pay a fee to lawfully perform his or her duties. See Mayo v. United States, 319 U.S. 441, 447, 63 S.Ct. 1137, 1140, 87 L.Ed. 1504 (1943) (holding that Supremacy Clause prohibits state from requiring United States to pay privilege tax before executing a function of government); Johnson v. Maryland, 254 U.S. 51, 57, 41 S.Ct. 16, 16-17, 65 L.Ed. 126 (1920) (holding that state may not require federal postal employee to obtain state driver’s license before performing official duties). Any *1573attempt by a state or local government to tell a federal judge what he or she must do to lawfully perform federal duties offends elemental notions of federal supremacy.16
In practice, any attempt to apply Ordinance No. 1120 to federal judges threatens to lead to clashing sovereignty. Enforcement of the privilege tax requirement against federal judges risks intrusion into a federal judge’s judicial affairs. To determine the amount of a federal judge’s privilege tax, Jefferson County must determine what percentage of the judge’s duties were performed in Jefferson County. We question whether a state or local government may inquire into precisely how and where a federal judge spends time on judicial duties; even if permissible, such an inquiry is apt to engender intergovernmental conflict. A further source of conflict is the practical effect of the privilege tax17 on federal judges’ willingness to sit or otherwise perform duties in Jefferson County.
We note that, in the performance of federal judicial duties, non-resident federal judges often are called upon to sit in Jefferson County. United States v. Tokars, 839 F.Supp. 1578 (N.D.Ga.1993), is just one example. Tokars was a federal criminal racketeering prosecution involving allegations that the murder of a young woman in front of her two children was committed by two hitmen hired by her husband, an Atlanta attorney. Atlanta, the case’s original venue, was saturated with publicity about the case. To safeguard the defendant’s constitutional right to a fair trial, a district judge for the Northern District of Georgia granted the defendant a change of venue and spent five weeks in Birmingham trying the case. Under Ordinance No. 1120, the Atlanta-based federal judge would owe Jefferson County a percentage of her salary because she chose Birmingham as the most appropriate venue where the accused could get a fair trial.18
C. Congressional Consent to State Taxation
Congress generally has the power to consent to state taxation of federal employees, operations, and instrumentalities. Mayo, 319 U.S. at 446, 63 S.Ct. at 1140. Jefferson County argues that Congress, in the Public Salary Act and the Buck Act, consented to all forms of state and local taxation of federal employees, including federal judges. Therefore, we examine whether the Public Salary Act and the Buck Act constitute consent to the imposition of the privilege tax on federal judges. The district court held that, under Article III, Congress may not consent to the imposition of the privilege tax on federal judges. Because we find that Congress did not consent to the imposition of the privilege tax on federal judges, we need not address Congress’s power to do so.
1. Public Salary Act
The Public Salary Act provides in relevant part:
The United States consents to the taxation of pay or compensation for personal service as an officer or employee of the United States, a territory or possession or political subdivision thereof, the government of the District of Columbia, or an agency or instrumentality of one or more of the foregoing, by a duly constituted taxing au*1574thority having jurisdiction, if the taxation does not discriminate against the officer or employee because of the source of the pay or compensation.
4 U.S.C. § 111. The Public Salary Act does not define the “taxation of pay or compensation for personal service” to which the United States consents. The County contends that Congress consented to the imposition on federal employees of all nondiscriminatory state and local taxes, including nondiscriminatory privilege taxes.
We do not interpret the Public Salary Act’s consent to state taxation of federal employees’ compensation as encompassing the imposition of privilege taxes such as Jefferson County’s. The Public Salary Act must be read in light of the uncertain state of the intergovernmental tax immunity doctrine at the time of the Act’s enactment. Before the Act was proposed, the Supreme Court held that the federal government could levy nondiscriminatory taxes on the incomes of state employees. Davis v. Michigan Dept. of Treasury, 489 U.S. 803, 811-814, 109 S.Ct. 1500, 1505-06, 103 L.Ed.2d 891 (1989) (describing context of Act’s enactment). The primary purpose of the Act was to amend the federal tax code to clarify that the federal income tax applied to the income of all state and local government employees. Id. at 811, 109 S.Ct. at 1505. See also H.R.Rep. No. 26, 76th Cong., 1st Sess., 3-4 (1939); S.Rep. No. 112, 76th Cong., 1st Sess. 11 (1939).
Congress was concerned, however, that considerations of fairness dictated equal tax treatment of federal and state employees. Davis, 489 U.S. at 812, 109 S.Ct. at 1506. The Supreme Court had decided Dravo but had not yet held in O’Keefe that the intergovernmental tax immunity doctrine does not bar states from taxing the income of federal employees. Thus, Congress entertained doubts about whether states could tax federal employees’ income without Congress’s consent. Id. at 811-812, 109 S.Ct. at 1506. To ensure equal tax treatment of all government employees, therefore, Congress decided to consent to state and local taxation of federal employees’ income. Id. at 812, 109 S.Ct. at 1506. Congress’s consent turned out to be unnecessary; O’Keefe was decided before the Act was enacted. Id. Congress nevertheless enacted the provision consenting to state and local taxation of federal employees’ compensation, effectively codifying the result in O’Keefe. Id.
The context of the Act’s enactment thus reveals that Congress intended to consent to state taxation of federal employees’ income to reciprocate for the imposition of the federal income tax on state employees. The Act does not consent to all state taxes on federal employees. We discern no congressional intent to consent to state taxes that in substance are not taxes on income. Thus, we interpret “taxation of pay or compensation for personal service,” 4 U.S.C. § 111, to refer to state taxes on income. The Public Salary Act does not alter the intergovernmental tax immunity doctrine; in effect, it just codifies the result in O’Keefe. Davis, 489 U.S. at 813, 109 S.Ct. at 1506.19
2. The Buck Act
The County also contends that Congress consented to taxes such as the Jefferson County privilege tax in the Buck Act, 4 U.S.C. §§ 106-110. The Buck Act provides in relevant part:
No person shall be relieved from liability for any income tax levied by any State, or by any duly constituted taxing authority therein, having jurisdiction to levy such a tax, by reason of his residing within a *1575Federal area or receiving income from transactions occurring or services performed in such area; and such State or taxing authority shall have full jurisdiction and power to levy and collect such tax in any Federal area within such State to the same extent and with the same effect as though such area was not a Federal area.
4 U.S.C. § 106(a). Unlike the Public Salary Act, the Buck Act defines the state taxation to which the United States consents. The Buck Act defines “income tax” as “any tax levied on, with respect to, or measured by, net income, gross income, or gross receipts.” Id. § 110(c).
The district court found that the privilege tax falls within the Buck Act’s definition of an “income tax” because the privilege tax is measured by gross receipts. We agree that the Buck Act’s definition of “income tax” encompasses the privilege tax. But another provision of the Buck Act removes the privilege tax from the Buck Act’s consent to state taxes. Echoing the intergovernmental tax immunity doctrine’s prohibition against state taxes levied directly on the federal government, the Buck Act provides that its provisions “shall not be deemed to authorize the levy or collection of any tax on or from the United States or any instrumentality thereof.” Id. § 107(a). According to the Supreme Court, “[t]his section can only be read as an explicit congressional preservation of federal immunity from state ... taxes unconstitutional under the immunity doctrine announced by Mr. Chief Justice Marshall in McCulloch v. Maryland.” State Tax Commission of Mississippi 421 U.S. at 612, 95 S.Ct. at 1880. Therefore, the Buck Act does not alter the intergovernmental tax immunity doctrine or constitute consent to the privilege tax.
Indeed, the Buck Act’s effect on the ability of states to tax federal employees is much more modest than Jefferson County suggests. According to its plain language, the Buck Act merely precludes a taxpayer from arguing that a state or locality lacks jurisdiction to tax her because she resides in a federal area or receives income from transactions or services in a federal area. 4 U.S.C. § 106(a). The Buck Act equalizes taxing power within and without federal areas, allowing states 'and localities to levy taxes within federal areas “to the same extent and with the same effect” as without federal areas. Id. The Buck Act does not, however, affect the limits' on state and local taxing power in any other way.20
The Supreme Court addressed the effect of the Buck Act on state and local taxation within federal areas in Howard v. Commissioners of Sinking Fund of City of Louisville, 344 U.S. 624, 73 S.Ct. 465, 97 L.Ed. 617 (1953). In Howard, employees of a naval ordnance plant located on federal land in Louisville, Kentucky, challenged the City of Louisville’s attempt to collect from them a license fee for the privilege of working in Louisville. Id. at 625, 73 S.Ct. at 466. The Supreme Court noted that the United States had exclusive jurisdiction over the federal area, except as modified by statute. Id. at 627, 73 S.Ct. at 467. The Court held that the license fee was an “income tax” under the Buck Act, id. at 629, 73 S.Ct. at 468, and that the Buck Act therefore granted Louisville the right to impose the license fee on the federal employees working at the ordnance plant. Id. at 628, 73 S.Ct. at 467. The Court explained, “By virtue of the Buck Act, the tax can be levied and collected within the federal area, just as if it'were not a federal area.” Id. at 629, 73 S.Ct. at 468.
*1576The County suggests that the Buck Act authorizes Jefferson County to levy its license fee on federal judges just as the Buck Act was held in Howard, to authorize Louisville to levy its license fee on federal employees of the ordnance plant. The challenge to the Jefferson County privilege tax, however, differs significantly from the challenge in Howard. Judges Acker and Clemon do not contend that Jefferson County may not tax them because they work within a federal area. Rather, they argue that, regardless of where in Jefferson County they perform their duties, Jefferson County may not levy the privilege tax on them because to do so would amount to a direct tax on instrumen-talities of the federal government in violation of the intergovernmental tax immunity doctrine. The federal employees in Howard, in contrast, did not contend that the license fee directly taxed the federal government. They challenged the license fee solely on the one ground barred by the Buck Act — that Louisville lacked jurisdiction to tax in a federal area — and the Supreme Court addressed only that ground. Thus, Howard does not address the issue presented here.
Nothing in Howard undermines our conclusion that the Buck Act does not alter the intergovernmental tax immunity doctrine’s limits on state and local taxation. Howard cannot be read, for example, as an implicit rejection of intergovernmental tax immunity from privilege taxes falling within the Buck Act’s definition of “income tax.” An intergovernmental tax immunity challenge, if raised by the Howard employees, would have failed not because the Buck Act precluded such a challenge but because the Louisville license fee did not amount to a direct tax on the federal government or its instrumentalities. Assuming that the taxed activity was working in Louisville, the Howard employees could not be considered the federal government or its instrumentalities when performing their duties. Unlike federal judges, employees of a naval ordnance plant realistically can be viewed as separate entities from the federal government when performing their duties; they are not “intimately connected with the exercise of a power or the performance of a duty by the Government.” New Mexico, 455 U.S. at 736, 102 S.Ct. at 1383. Thus, that Howard upheld the application of the Louisville license fee to federal employees does not imply that the Buck Act precludes an intergovernmental tax immunity challenge to the application of Ordinance No. 1120 to federal judges.
V. CONCLUSION
As applied to federal judges, the privilege tax violates the intergovernmental tax immunity doctrine as a direct tax on the federal government or its instrumentalities. We hold, therefore, that the Supremacy Clause prohibits Jefferson County from applying Ordinance No. 1120 to Judges Acker and Clem-on.
AFFIRMED.
. Article III of the Constitution vests the judicial power of the United States in the Supreme Court “and in such inferior Courts as the Congress may from time to time ordain and establish." U.S. Const, art. Ill, § 1. Article III judges include federal district court judges, judges for the circuit courts of appeals, and justices of the Supreme Court.
. Given the nature of the question presented in this case, we considered the issue of recusal at the outset. Our discussion of the recusal issue is included as an appendix.
. The ordinance also includes the following definition of "vocation, occupation, calling and profession”:
The words “vocation, occupation, calling and profession” shall mean and include the doing of any kind of work, the rendering of any kind of personal services, or the holding of any kind of position or job within Jefferson County, Alabama, by any clerk, laborer, tradesman, manager, official or other employee, including any non-resident of Jefferson County who is employed by any employer as defined in this section, where the relationship between the individual performing the services and the person for whom such services are rendered is, as to those services, the legal relationship of employer and employee, including also a partner of a firm or an officer of a firm or corporation, if such partner or officer receives a salary for his personal services rendered in the business of such firm or corporation, but they shall not mean or include domestic servants employed in private homes and shall not include businesses, professions or occupations for which license fees are required to be paid under any General License Code of the County or to the State of Alabama or the County by any of the following [listing sections of the Code of Alabama].
Ordinance No. 1120, § 1(B).
. Ordinance No. 1120, § 1(F) provides:
The words "gross receipts” and “compensation" shall have the same meaning, and both words shall mean and include the total gross amount of all salaries, wages, commissions, bonuses or other money payment of any kind, or any other considerations having monetary value, which a person receives from or is entitled to receive from or be given credit for by his employer for any work done or personal services rendered in any vocation, occupation, calling or profession, including any kind of deductions before "take home” pay is received
. Each day one works without a license constitutes a separate offense. Alabama Code § 11-51-93 (1989).
. The Northern District of Alabama holds court in both Birmingham and Gadsden. 28 U.S.C. § 81 (a)(3) and (6).
. The Honorable Charles A. Moye, Jr., U.S. District Judge for the Northern District of Georgia, sitting by designation.
. The Compensation Clause provides that Article III judges “shall, at stated Times, receive for their Services, a Compensation, which shall not be diminished during their Continuance in Office.” U.S. Const, art. Ill, § 1.
. The district court also held that the ordinance does not discriminate against Judges Acker and Clemon by reason of the federal source of their compensation in violation of the Public Salary Act, 4 U.S.C. § 111. On this appeal, there is no contention that this holding was erroneous and, in light of our disposition of the case, we do not address it.
. The County recognizes that the intergovernmental tax immunity doctrine also bars taxes that discriminate against the federal government. But the dispute on this appeal does not center on whether the privilege tax is discriminatory and, in light of our disposition of the case, we do not address whether the privilege tax is discriminatory-
. Purporting to eschew the' economic-burden theory, some litigants have couched their arguments simply in terms of interfering with federal functions, but these challenges invariably have amounted to challenges to the tax’s economic burden.
. Attorneys, for example, must pay a flat annual license fee of $250 to the state, regardless of their income. Ala.Code § 40-12-49,
. The ordinance imposes withholding requirements on employers, but contemplates that the license fee will be paid by the person engaging in the work.
. The ordinance is not backed by criminal penalties, the County argues, so it is "unlawful" to work without paying the privilege tax only in the sense that it is "unlawful" to refuse to pay any civil debt.
. At oral argument, counsel for Jefferson County stated that the County appears to have copied Birmingham’s privilege tax ordinance verbatim. Under Alabama law, a city, unlike a county, does have the power to criminally prosecute and punish violators of a license tax ordinance. Ala. Code! 11-51-93.
.The Supreme Court has described the freedom of the federal courts from state interference, albeit in a different context, in this way:
It may not be doubted that the judicial power of the United States as created by the Constitution ... is a power wholly independent of state action, and which therefore the several states may not by any exertion of authority in any form, directly or indirectly, destroy, abridge, limit, or render inefficacious. The doctrine is so elementary as to require no citation of authority to sustain it. Indeed, it stands out so plainly as one of the essential and fundamental conceptions upon which our constitutional system rests, and the lines which define it are so broad and so obvious, that ... the attempts to transgress or forget them have been so infrequent as to call for few occasions for their statement and application.
Harrison v. St. Louis & San Francisco R.R. Co., 232 U.S. 318, 328, 34 S.Ct. 333, 335, 58 L.Ed. 621 (1914).
. The effect includes the burden of recordkeep-ing and disclosure requirements.
. When questioned at oral argument about whether the Tokars judge owes the privilege tax for trying the case in Birmingham, counsel for Jefferson County replied: “Under ordinance yes, I believe she does, I believe she does.”
. Our interpretation of the Public Salary Act as consenting only to taxes that in substance tax income is not inconsistent with the Third Circuit’s decision in United States v. City of Pittsburgh, 757 F.2d 43 (3rd Cir.1985). Adopting a broad reading of "taxation of pay or compensation,” the Third Circuit held that the Public Salary Act consented to Pittsburgh's levy of a privilege tax on a court reporter's transcript fee income. Id. at 47. Unlike the Jefferson County privilege tax, the Pittsburgh privilege tax was in substance a tax on income. The Third Circuit found that, despite its "privilege tax” label, the Pittsburgh tax was "clearly a tax on gross receipts or gross income from the fees.” Id. Though the Third Circuit did not discuss how it arrived at that conclusion, our examination of the Pittsburgh ordinance reveals that the ordinance did not include the factors that distinguish the Jefferson County ordinance from an income tax. See Pittsburgh, Pa., Ordinance No. 675 (Dec. 27, 1968).
. The Buck Act was enacted in 1940 against the background of the just-enacted Public Salary Act. The Public Salary Act's consent to state income taxes failed to reach federal employees residing and working in federal areas because, without congressional consent, the states lacked jurisdiction to tax transactions occurring in federal areas. United States v. Lewisburg Area Sch. Dist., 539 F.2d 301, 309 (3rd Cir.1976); United States v. City and County of Denver, 573 F.Supp. 686, 691 (D.Colo.1983) (citing S.Rep. No. 1625, 76th Cong., 3d Sess. 3 (1940)). The Buck Act therefore was enacted to eliminate the disparity between the income tax liability of federal employees within federal areas and those outside federal areas. Lewisburg Area Sch. Dist., 539 F.2d at 309; City and County of Denver, 573 F.Supp. at 691. It does so by eliminating immunity based solely on the ground that the taxpayer resides in a federal area or receives income from transactions or services in a federal area. The Act does not affect claims of tax immunity based on other grounds. See S.Rep. No. 1625, 76th Cong., 3d Sess. 2-3 (1940).