LAST week Colorado and Washington passed ballot initiatives to legalise recreational marijuana use. But, as John Hickenlooper, Colorado’s governor, warned, neither Coloradans nor Washingtonians should break out the Cheetos and Goldfish just yet. Whether they can actually implement these laws remains unclear. Marijuana possession remains illegal under federal law, and the federal government clearly has the power to enforce in those states. In California, for example, medical marijuana is legal, but the federal Drug Enforcement Agency (DEA) arrested nearly 8,500 people for marijuana-related offenses between 2004 and 2010. The Department of Justice has yet to say whether they will take a similar approach in Colorado and Washington, only that they are assessing the new laws.

But just because the federal government can do something, it does not mean it must. A coalition of unlikely allies on the left and right is urging the Obama administration to leave Colorado and Washington alone. Ron Paul, a Republican congressman from Texas, and Barney Frank, a liberal from Massachusetts (who, like Mr Paul, is retiring from Congress) sent a letter to Barack Obama urging him not to prosecute Coloradans or Washingtonians.

There are good reasons for Mr Obama to heed that advice, besides respect for personal freedom. For one thing, prosecuting people for marijuana offenses is a waste of resources. According to the most recent data available, 12.4% of federal prisoners convicted on drug charges were locked up for marijuana offenses. That’s about 11,630 people, at an average cost of $25,500 to $26,000 per person per year. For another, as Mark Kleiman sensibly argues, allowing Colorado and Washington to try out their new legal regimes will amount to a controlled experiment that will provide valuable comparative data. Louis Brandeis, a former Supreme Court justice, called states “laboratories of democracy”, a phrase that applies almost literally to the situation in which Colorado and Washington find themselves today.

Still, “should” is one thing, “will” another. But here too, Mr Kleiman and those who take his position have cause for hope. It is one thing for states to pass laws legalising marijuana for medical reasons, quite another for them to effectively announce that they will no longer enforce marijuana-prohibition on small-scale users. With the latter there is no winking, no fig leaves, no need to keep up appearances. Also, ending prohibition has the electoral wind at its back. In two of three states in which legalisation was on the ballot, it passed (the failed measure in Oregon was more far-reaching than the other two). In Washington, legalisation proponents outraised opponents 400-fold. Police in those two states have decided they have better things to do than bust people for having a little weed in their pockets. Eric Holder, Mr Obama’s attorney general, may well decide that federal law-enforcement officers do too.