Like the Uniformity Clause, with which it was initially joined at the Constitutional Convention, and the Export Taxation Clause, the Port Preference Clause was meant to interfere with the natural tendency of legislatures to become instruments through which powerful commercial interests injure their politically weaker rivals.

The impetus for the Port Preference Clause came from the Maryland delegation, whose members were especially worried that vessels bound to or from the port of Baltimore might be required to stop in Virginia. Some other delegates objected that Congress should not have its hands tied, lest it be unable to deal adequately with problems such as smuggling on long rivers like the Delaware. The issue was referred to a committee, which included a delegate from each state, and which recommended language nearly identical to the final version now in the Constitution.This was not sufficient for Maryland’s Luther Martin, who became a leading Anti-Federalist. He objected that Congress might easily violate the spirit of the provision, perhaps by limiting Maryland to one inappropriate port of entry on the Potomac: this would effectively require Baltimore shipping to stop in Virginia.

The language of the Port Preference Clause sweeps beyond the specific concerns that motivated its proponents at the Convention. The Supreme Court, however, has construed the Port Preference Clause very narrowly, holding that Congress may grant enormous “incidental” preferences to the ports of certain states through devices such as making improvements (like dredging) or creating obstructions (like bridges) in one place rather than another. Pennsylvania v. Wheeling & Belmont Bridge Co. (1856) (“Wheeling Bridge II”); South Carolina v. Georgia (1876). The Court has indicated that the clause would be violated by naked discrimination between all the ports of one state and those of another. Wheeling Bridge II. But even this prohibition is essentially toothless: it has been read to allow Congress to impose a tax that affected all the ports of some states and no ports in some others. Augusta Towing Co., Inc. v. United States (1984).

A dissent in the seminal Supreme Court case complained that the majority’s interpretation rendered the clause a dead letter. Wheeling Bridge II(McLean, J., dissenting). More recently, Justice Clarence Thomas suggested in a concurrence that a natural reading of the constitutional language “prohibits Congress from using its commerce power to channel commerce through certain favored ports.” United States v. Lopez (1995) (Thomas, J., concurring). As the case law stands, however, Congress is only on its honor to comply with the spirit of the clause by refraining from politically motivated favoritism that distorts the natural economic competition among American ports.