Yesterday’s unanimous Supreme Court opinion in American Trucking Associations v. City of Los Angeles is a run-of-the-mill federal preemption case, not inviting much attention. But the interesting bit isn’t Justice Kagan’s majority opinion. It’s Justice Thomas’s short concurrence. Thomas agrees that federal law trumps conflicting state/local law regarding certain regulations related to the Port of Los Angeles, but seizes on the plain language of the preempting statute to take a shot at the massive expansion of federal authority under a misreading of the Commerce Clause.

Justice Thomas focuses on a section of the relevant statute (the Federal Aviation Administration Authorization Act, or FAAAA–don’t ask why this covers ports) titled “Federal authority over intrastate transportation.” He denies that Congress possesses this authority: the Commerce Clause, part of Article I, section 8, only gives Congress the power to regulate commerce “among the several States.” Thomas can’t believe that Congress could have been granted power to legislate something so local as where trucks park once they leave the port (one of the regulations at issue in American Trucking):

Congress cannot pre-empt a state law merely by promulgating a conflicting statute–the preempting statute must also be constitutional, both on its face and as applied. As relevant here, if Congress lacks authority to enact a law regulating a particular intrastate activity, it follows that Congress also lacks authority to pre-empt state laws regulating that activity

The reason that Justice Thomas nevertheless concurs in the judgment here, however, is that Los Angeles waived any constitutional claims against the FAAAA, instead relying solely on statutory arguments (which correctly lost 9-0).

This isn’t the first time that Thomas upheld a federal law but noted federalism concerns that, as here, the plaintiffs didn’t raise (or didn’t preserve on appeal). In Gonzales v. Carhart, for example, Thomas concurred with a majority decision that sustained the federal Partial-Birth Abortion Ban Act against a challenge based on Roe v. Wade and Planned Parenthood v. Casey but noted that the issue of whether a federal abortion regulation “constitutes a permissible exercise of Congress’ power under the Commerce Clause is not before the Court. The parties did not raise or brief that issue; it is outside the question presented; and the lower courts did not address it.”

Justice Thomas’s opinions in these sorts of cases illustrate the misuse of the Commerce Clause given the Constitution’s careful enumeration of congressional powers. These brief, pointed concurrences show that our imperial government isn’t clothed in constitutional authority.

And they also have a direct use for legal practitioners. I wasn’t a “real” lawyer for that long before joining Cato, but here’s an easy practice tip: Don’t just assume that the federal government has the power to pass the law you don’t want applied to your client.