Public opinion on cannabis legalization has been trending positive since sometime in the late 1990s. By a couple of years ago, it was just about break-even. But in the latest Gallup poll, the “pro” side has moved to an almost-20-point advantage, 58 to 39.

Of course this calls for some caveats.

1. The question, as asked, was about legalizing use, not production and sale.

2. With a sample size of only 1000, some of the movement might be noise rather than signal. The Galston-Dionne analysis of Pew Center data from earlier in the year shows a more closely divided country; again, it’s an open question whether that difference is measurement error or real change over a few months.

3. Bad outcomes in Washington or Colorado might reverse the trend.

All of that said, the new numbers are enough to move my probability distribution. For a while now, I’ve been predicting national legalization sometime in Hillary Clinton’s second term, but without much conviction and with the caveat that the trend in public opinion would have to continue in order to make that prediction come true. Now I’d have to say it will come true unless the trend were somehow to reverse.

Note also that at some point positive feedback takes over. Being for pot legalization has a different social meaning when it’s the clear majority view. The polling results will therefore influence the behavior of some survey respondents and voters directly. Moreover, politicians and other opinion leaders shift their announced (and perhaps actual) views in response to perceived shifts in voter attitudes, that in turn will shift the beliefs of some voters.

If the question of whether to legalize now seems largely settled, that makes the much-less-debated question of how to legalize even more topical. Some of the smarter opponents of cannabis have figured this out, and are now looking for ways of limiting the increase in drug abuse likely to follow legal availability. However, career and ideological interests and group ties are likely to lead the majority of the active drug warriors to keep fighting what now seems like an unwinnable battle, telling one another that legalization is sure to be such a disaster that the public will demand re-prohibition. By doing so, the warriors will help to ensure that the legal system that eventually arises will be over-commercialized, under-regulated, and under-taxed.

This would simply repeat the mistake they made in opposing the medical use of cannabis. While the warriors kept chanting “Cheech and Chong medicine,” the pot advocates rolled right over them. Rien appris, rien oublié.

Update Andrew Sullivan strikes a triumphal note. Hard to fault him for that. But goddammit, “less harmful than alcohol” and “not harmful to most of its users” do not add up to “harmless.” Adolescents spin out on cannabis and wreck their academic careers. People of all ages do stupid things while stoned, including driving their cars into trees and other cars. Cannabis now follows only alcohol as the primary drug of abuse reported by people voluntarily entering drug treatment.

Why take the perfectly reasonable case that cannabis should not be illegal and ruin it with the silly claim that the stuff is harmless?

[Cross-posted at The Reality-Based Community]

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Mark Kleiman is a professor of public policy at the New York University Marron Institute.