At the end of May, Colorado Gov. John Hickenlooper signed legislation aimed at taxing and regulating the commercial distribution of marijuana for recreational use. The legislative process had been haunted by the fear that the federal government would try to quash this momentous experiment in pharmacological tolerance—a fear magnified by the Obama administration's continuing silence on the subject.

More than half a year after voters in Colorado and Washington made history by voting to legalize marijuana, Attorney General Eric Holder still has not said how the Justice Department plans to respond. If the feds are smart, they will not just refrain from interfering; they will work together with state officials to minimize smuggling of newly legal marijuana to jurisdictions that continue to treat it as contraband. A federal crackdown can only make the situation worse—for prohibitionists as well as consumers.

Shutting down state-licensed pot stores probably would not be very hard. A few well-placed letters threatening forfeiture and prosecution would do the trick for all but the bravest cannabis entrepreneurs. But what then?

Under Amendment 64, the Colorado initiative, people 21 or older already are allowed to possess up to an ounce of marijuana, grow up to six plants for personal use, and keep the produce of those plants (potentially a lot more than an ounce) on the premises where they are grown. It is also legal to transfer up to an ounce "without remuneration" and to "assist" others in growing and consuming marijuana. Put those provisions together, and you have permission for various cooperative arrangements that can serve as alternative sources of marijuana should the feds stop pot stores from operating.

Washington's initiative, I-502, does not allow home cultivation. But University of California at Los Angeles drug policy expert Mark Kleiman, who is advising the Washington State Liquor Control Board on how to regulate the cannabis industry, argues that collectives ostensibly organized to serve patients under that state's medical marijuana law could fill the supply gap if pot stores never open. It is also possible that Washington's legislature would respond to federal meddling by letting people grow marijuana for personal use.

With pot shops offering a decent selection at reasonable prices, these alternative suppliers will account for a tiny share of the marijuana market, just as home brewing accounts for a tiny share of the beer market. But if federal drug warriors prevent those stores from operating, they will be confronted by myriad unregulated, small-scale growers, who will be a lot harder to identify, let alone control, than a few highly visible, state-licensed businesses.

The feds, who account for only 1 percent of marijuana arrests, simply do not have the manpower to go after all those growers. Nor do they have the constitutional authority to demand assistance from state and local law enforcement agencies that no longer treat pot growing as a crime.

Given this reality, legal analyst Stuart Taylor argues in a recent Brookings Institution paper, administration and state officials should "hammer out clear, contractual cooperation agreements so that state-regulated marijuana businesses will know what they can and cannot safely do." Such enforcement agreements, which are authorized by the Controlled Substances Act, would provide more security than a mere policy statement, although less than congressional legislation.

Taylor, who says he has no firm views on the merits of legalization, warns that "a federal crackdown would backfire by producing an atomized, anarchic, state-legalized but unregulated marijuana market that federal drug enforcers could neither contain nor force the states to contain." He says the public debate over that issue would benefit from evidence generated by the experiments in Colorado and Washington. That's assuming the feds do not go on a senseless rampage through these laboratories of democracy.