In an interview with NPR's Scott Simon yesterday, former Supreme Court Justice John Paul Stevens, who had previously expressed support for allowing medical use of marijuana, endorsed general legalization:

Simon: An increasing number of states are legalizing marijuana. Should federal law? Stevens: Yes. Simon: We may have just made some news. Stevens: Yes. I really think that that's another instance of public opinion [that's] changed and recognized that the distinction between marijuana and alcoholic beverages is really not much of a distinction. Alcohol, the prohibition against selling and dispensing alcoholic beverages, has I think been generally—there's a general consensus that it was not worth the cost. And I think really in time that will be the general consensus with respect to this particular drug.

That answer makes Stevens the first current or former member of the Supreme Court to publicly support the repeal of pot prohibition. Good for him. But Stevens might want to reflect a bit on his support for the broad view of congressional power that made pot prohibition possible, especially since he mentions the experience with alcohol prohibition. As you may recall from history books and as Stevens (who turned 94 on Sunday) may recall from his own childhood, banning alcohol at the national level required a constitutional amendment, and so did repealing it. Why? Because everyone recognized that the Constitution did not authorize Congress to impose its pharmacological preferences on the entire country. Yet the Constitution was never amended to authorize marijuana prohibition, which was initially disguised as a revenue measure and today is based on a reading of the Commerce Clause broad enough to accommodate almost any legislative whim.

It was the Commerce Clause that the Supreme Court cited in 2005, when it ruled in Gonzales v. Raich that that the feds could arrest, prosecute, and imprison people who grow and possess marijuana in compliance with state laws allowing medical use. According to Stevens, who wrote the majority opinion, it did not matter if the marijuana never crossed state lines: The tiniest speck of cannabis, anywhere within the country's borders, is covered by the power to "regulate commerce…among the several states." That conclusion, Stevens said, followed logically from Wickard v. Filburn, the 1942 case in which the Court had upheld enforcement of agricultural quotas based on the Commerce Clause. If the Commerce Clause allows the federal government to punish a farmer for growing wheat that never leaves his farm, Stevens reasoned, surely it allows the federal government to punish a cancer patient for growing or possessing marijuana that never leaves her home. "The case comes down to the claim that a locally cultivated product that is used domestically rather than sold on the open market is not subject to federal regulation," Stevens wrote. "Wickard and its progeny foreclose that claim."

Justice Clarence Thomas, no fan of Wickard, saw the issue differently. "If Congress can regulate this under the Commerce Clause," Thomas wrote in his dissent, "then it can regulate virtually anything—and the Federal Government is no longer one of limited and enumerated powers." That prospect probably did not worry Stevens, who dissented whenever the Court attempted to enforce even modest limits on the scope of the Commerce Clause. In the 1995 case United States v. Lopez, for instance, he said a federal ban on the possession of guns in or near schools should be upheld because "the welfare of our future 'Commerce with foreign Nations, and among the several States,' is vitally dependent on the character of the education of our children."

You could credit Stevens with principled consistency, since his opinion upholding the federal ban on marijuana apparently went against his own policy preferences. But what exactly is the principle that Stevens is defending? That Congress can do whatever it wants, as long as it's not explicitly prohibited by the Constitution? As Thomas observed, that approach negates the principle of enumerated powers, the bedrock of federalism. And if the Constitution can be changed so dramatically without amendment—such that today it authorizes a policy, national marijuana prohibition, that it did not authorize prior to 1942—even the explicit limits on government power are vulnerable, as Stevens himself has demonstrated in cases involving the First Amendment, Second Amendment, Fourth Amendment, and Fifth Amendment.

In fact, Stevens has been a principal enabler of the war on drugs, not just in Raich but in a string of search and seizure cases that have whittled away at the Fourth Amendment in the name of stopping Americans from consuming substances that legislators do not like. Now he says the war on drugs (or a major part of it, at least) was a big mistake. One of the Constitution's virtues is that it can save us from mistakes like that, provided courts are prepared to enforce it.