...all Duties, Imposts and Excises shall be uniform throughout the United States....
Among the unsatisfactory aspects of the Confederation government were its inability to regulate interstate and foreign commerce and its weak powers of taxation. The Constitution cured these defects, but thereby created a new danger: the greatly strengthened national government might abuse its powers by oppressing politically weaker groups and strangling the economic activity that the Framers hoped to promote.
At the Constitutional Convention, the Uniformity Clause was initially joined with what is now the Port Preference Clause (Article I, Section 9, Clause 6), which forbids Congress to give preferences "by any Regulation of Commerce or Revenue" to the ports of one state over those of another. Along with other provisions restricting congressional power over taxes and commercial regulations, these two were designed to forestall economically oppressive discrimination. The Port Preference Clause limits both the commerce and taxing powers, whereas the Uniformity Clause applies to the taxing power alone. Their common origin, however, is a sign of their common purpose: each was meant to prevent geographic discrimination that would give one state or region a competitive advantage or disadvantage in its commercial relations with the others.
Because the goods and activities that can be taxed are distributed unequally through the country, virtually all duties, imposts, and excises have nonuniform effects. A tax on oil production, for example, will affect certain regions more severely than others. Because the Constitution expressly empowers Congress to levy these taxes, it must also permit some of the nonuniform effects that inevitably accompany them. The principal challenge in interpreting the Uniformity Clause is to distinguish between the kind of nonuniformity that is forbidden by the Constitution and the inevitable nonuniform effects that accompany legitimate duties, imposts, and excises.
In its earliest exposition, the Supreme Court declared that a tax is uniform if it "operates with the same force and effect in every place where the subject of it is found." Edye v. Robertson (1884). This rule correctly recognized that the Uniformity Clause was meant to forbid geographically nonuniform taxes without outlawing all geographically nonuniform effects. But the formula is inadequate, because it does not describe the limits on Congress's discretion to define the "subjects" of taxation. Suppose, for example, that Congress chose to define the subject of an excise tax as "oil produced in Alaska." The rule would be formally satisfied, but the most flagrant geographic discrimination would be possible.
In United States v. Ptasynski (1983), a unanimous Court concluded (1) that any tax in which the subject is defined in nongeographic terms satisfies the Uniformity Clause, and (2) that where the subject is defined in geographic terms, the tax will be scrutinized for "actual geographic discrimination."
The first part of this test creates a very large safe harbor for discriminatory taxes, which can almost always be framed without using overtly geographic terminology (for example, "oil whose production might affect caribou populations"). Nor is it clear that the second part of the test puts any real limit on Congress's power to impose discriminatory and oppressive taxes, for the Court nowhere defined "actual geographic discrimination." In fact, the Court went out of its way to emphasize that review of statutes using geographic terminology would be highly deferential. With no promise of effective judicial enforcement, the Uniformity Clause has, at least for the present, apparently been rendered nugatory, save for Congress's own sense of its obligations under the Constitution.
Richard A. Epstein, Takings: Private Property and the Power of Eminent Domain 289–293 (1985)
Nelson Lund, Comment, The Uniformity Clause, 51 U. Chi. L. Rev. 1193 (1984)
Edye v. Robertson, 112 U.S. 580 (1884)
Knowlton v. Moore, 178 U.S. 41 (1900)
United States v. Ptasynski, 462 U.S. 74 (1983)