On May 27 Arizona Gov. Jan Brewer asked a federal judge to decide whether her state’s Medical Marijuana Act, narrowly approved by voters last November, “complies with federal law” or is “preempted in whole or in part because of an irreconcilable conflict with federal law.” Oddly, Brewer expressed no preference between those two diametrically opposed choices, which reinforced the impression that her suit was a veiled attempt to overturn Arizona’s law without antagonizing its supporters.

Whatever her motives, Brewer, who opposed the initiative before the election, raised questions that have important implications for the future of federalism in America, especially in the area of drug policy, where almost all of the encouraging action is happening at the state level. But the outcome she seemed to prefer—nullification of the Arizona Medical Marijuana Act by the federal Controlled Substances Act—would be a severe blow to state autonomy, a constitutional principle she zealously defends in other contexts.

As a legal matter, it’s clear the Justice Department can prosecute state-licensed suppliers of medical marijuana if it wants to; it can even go after the patients and caregivers that it says need not fear federal charges. In the 2001 case U.S. v. Oakland Cannabis Buyers’ Cooperative, the U.S. Supreme Court ruled that federal law does not recognize a medical necessity defense against prosecution under the Controlled Substances Act, which bans marijuana for all purposes. Four years later, in Gonzales v. Raich, the Court said the federal government’s power to “regulate commerce…among the several states” extends to the tiniest speck of marijuana wherever it may be found, even in the home of a woman who grows it for her own medical use in compliance with state law.

Based on this absolute federal ban, drug warriors claim state laws allowing marijuana use, whether for medical or recreational purposes, violate the Supremacy Clause, which says “this Constitution, and the laws of the United States which shall be made in pursuance thereof…shall be the supreme law of the land.” The nation’s experience with alcohol prohibition refutes this argument. Even under a prohibition system that, unlike the current one, was explicitly authorized by the Constitution (via the 18th Amendment), states had no obligation to ban what Congress banned or to punish what Congress punished. In fact, state and local resistance to alcohol prohibition led the way to national repeal.

From the beginning, alcohol prohibition was resented in America’s wetter provinces, where the authorities often declined to enforce it. Maryland never passed its own version of the Volstead Act, while New York repealed its alcohol prohibition law in 1923. Eleven other states eliminated their statutes by referendum in November 1932, months before Congress presented the 21st Amendment (which repealed the 18th) and more than a year before it was ratified.

In the 1996 case Printz v. United States, which overturned provisions of the Brady Handgun Violence Prevention Act that commanded local law enforcement officials to do background checks on gun buyers, the Supreme Court reaffirmed that “even where Congress has the authority…to pass laws requiring or prohibiting certain acts, it lacks the power directly to compel the States to require or prohibit those acts.” Hence the federal government cannot require states to establish and enforce their own penalties for marijuana cultivation, distribution, and possession, let alone insist that they allow no exceptions.

Although Brewer claimed to be worried that the federal government could prosecute state employees for licensing and regulating dispensaries under the Arizona Medical Marijuana Act, Dennis Burke, the U.S. attorney for Arizona, said he had “no intention” of doing that. In any case, all state regulators would be doing is determining who qualifies for a medical exemption from state drug penalties. As the American Civil Liberties Union noted in a motion seeking dismissal of Brewer’s suit, performing that function does not conflict with the Controlled Substances Act or prevent the federal government from enforcing it.

The ACLU argues that there are no plausible grounds for charging state employees with a federal crime, since licensing and regulating dispensaries do not involve growing or distributing marijuana and do not meet the intent and knowledge requirements for convicting someone of conspiracy, aiding and abetting, acting as an accessory, or money laundering. The group adds that regulators could not be prosecuted simply for failing to rat out licensees to the feds, since “respecting confidentiality does not constitute an affirmative act,” which is required to convict someone of concealing a felony.

In short, although the Supreme Court says states cannot carve out exceptions to the Controlled Substances Act, that does not mean they cannot carve out exceptions to their own drug laws (or repeal them entirely). Under the Court’s absurdly broad reading of the Commerce Clause, Congress can decide to treat just about anything as a crime. But states are still free to disagree.  

Senior Editor Jacob Sullum is the author of Saying Yes: In Defense of Drug Use (Tarcher Penguin).