ON TUESDAY, Beto O'Rourke, a former city councilman from El Paso, defeated the longtime incumbent Silvestre Reyes in the Democratic primary for Texas's 16th congressional district. It was probably the biggest upset in the state, and an outcome that has attracted national attention, for a simple reason: Mr O'Rourke, who will almost certainly win the general election in November, supports legalising marijuana.

While not entirely unprecedented, this is an outlying opinion among politicians. Polling shows that fully half of Americans now support legalising marijuana. Yet among national office-holders, the figure is about 0-1%. As Paul Waldman argues, the disparity might arise from the fact that there aren't really any electoral incentives for the politician who wants to go to bat on this issue, but there are plenty of risks—the risk of being seen as soft on crime, the risk of being seen as a crank, etc. Mr O'Rourke is perhaps insulated from these risks, because this is manifestly an issue that affects the district he hopes to represent, rather than some kind of dilettantish libertarian thing. He and Susie Byrd, also a former city representative, published a book last year describing the devastation of Mexico's drug war, particularly in El Paso's twinned city of Juarez, and arguing that decriminalising marijuana would be the best way to dismantle the black market that fuels the trade. The Economist supports decriminalising drugs for similar reasons, and such arguments are more compelling than complaints about personal freedom which, while valid, can come across as tasteless and self-absorbed. You can't open a paper from Juarez without reading about somebody being beheaded or disemboweled.

It would be wrong to interpret the race as an up-and-down vote on drug policy: as suggested before, this was a local election between two prominent local politicians, and the outcome might have hinged on turnout. And it would be surprising if a freshman representative single-handedly changed America's drug policy. Still, Mr O'Rourke's win is significant. The debate over decriminalising marijuana might have similar contours as the debate over legalising gay marriage, albeit for different reasons. Ten years ago, that is, gay marriage was widely seen as an extremely marginal issue, if it was seen at all; five years ago it was a mainstream but controversial issue; today, a majority of Americans are in favour, and top-level politicians (often a lagging indicator of social change) are coming out in favour of the cause too. At every step of the way, proponents helped their friends and neighbours get used to the idea, not just by making reasoned arguments, but by serving as living proof that the cause in question was not alien. With regard to drug policy, having elected officials who support decriminalisation or other alternatives to the war on drugs means that the spectrum of mainstream public opinion is expanding. That will make it easier for others to come around too.