Medical marijuana is legal in 23 states, but the drug remains illegal under federal law. Imagine a retired grandmother who is suffering terribly with a serious illness. Her doctor believes that marijuana might help. Her neighbors don't mind if she fills a pot prescription: They overwhelmingly voted to give her that right. Sure enough, the woman finds that smoking weed lessens her suffering. Should the federal government be empowered to arrest her for consuming it?

Many in Congress think so. And while federal agents are unlikely to intervene in this sort of case because the optics would be so awful, the law allows for it.

But Senator Rand Paul, a Republican, and his Democratic colleagues, Senators Cory Booker and Kirsten Gillibrand, hope a legislative majority will endorse a less-callous approach. Tuesday, they are introducing a bipartisan bill that would "allow patients, doctors and businesses in states that have already passed medical-marijuana laws to participate in those programs without fear of federal prosecution."

The bill is expected to divide Republicans. Senator Paul, his colleague Ted Cruz, and Governor Rick Perry, among others, have some regard for the 10th Amendment, which states that "the powers not delegated to the United States by the Constitution, nor prohibited by it to the states, are reserved to the states respectively, or to the people." They also believe that states can act as laboratories of democracy: People in different regions can live under a system of their liking while acting as a model for other Americans as they weigh the best approach.