When Colorado voted to legalize marijuana in 2012, Republican Cory Gardner was on the losing side. As a member of the U.S. House of Representatives, Gardner rebuked Eric Holder, the attorney general under President Barack Obama, for making the enforcement of federal marijuana laws in legalizing states a low priority. Now that he is a senator, however, Gardner has become convinced that states should be able to set their own marijuana policies.

He has shown some of the zeal of the convert. After Attorney General Jeff Sessions ended Holder’s policy, Gardner said he would block appointments to the Justice Department until it was reinstated. I criticized Gardner at the time, on the ground that a senator who has a problem with federal law should work to change it instead of demanding that the federal government stop enforcing it in certain jurisdictions.

Now he is proposing to change the law. He is part of a group of four Republicans and four Democrats behind a bill that declares that federal laws against “marihuana” do not apply in states where its production, distribution, and use is legal. (The archaic spelling is the one used in the federal statutes.) The lead sponsor is Senator Elizabeth Warren, the Massachusetts Democrat. Half of the cosponsors, including Gardner, represent legalizing states. President Donald Trump has said he will probably support the bill.

A pure federalist policy would go further. It would leave it up to states to police marijuana-related activities within their borders — all states, not just the ones that have legalized marijuana. The federal role would be limited to preventing interstate commerce from undermining state laws.

In practice, though, the federal government has never taken the lead role in suppressing all of the marijuana trade it purports to ban. States and localities do most of the work. That’s why states such as Colorado pose such a problem for drug warriors such as Sessions. The federal government lacks both the capacity and the willingness to enforce its marijuana laws by itself.

The new bill moves the U.S. closer to a federalist division of labor on marijuana, with states setting the policy and the federal government helping states that want assistance. States would be able to adopt varied approaches, and citizens would be able to compare the results. I’d like to see a state try legalizing marijuana without legalizing a for-profit marijuana industry. If a state’s voters disliked the effects of their policies, they would know which officials to hold accountable.

There is also the matter of the Constitution, with which a federal “war on drugs” is an uncomfortable fit. In 2005, three justices of the Supreme Court voted that, at least as applied to medical marijuana, that war exceeded the legitimate powers of the federal government. One of the justices who voted the other way, the late Antonin Scalia, argued that Congress could “reasonably conclude” that it had to prohibit the intrastate use of marijuana to keep an interstate market in the drug from developing. He did not deny that Congress could reasonably reach a different conclusion. Senators Warren, Gardner and the others are moving toward a better one.