I said over at PP yesterday that I think Democrats have a fight coming over marijuana, thanks to new polling that shows a majority currently favor legalization.
I compared the situation to marriage, and I see Joshua Tucker did, too; in many ways it's a natural comparison.
I do have one caution about it: I'm a lot less confident that it's a one-direction lasting trend than the marriage trend. This is basically just speculative, but for whatever it's worth...
I've always been fairly confident that the case against same-sex marriage would dissolve if and when it was enacted. The arguments against it often boiled down to "that's weird," and if and when it was enacted, it would rapidly no longer be weird. Sure, there's also some explicit bigotry, but for some time it was pretty clear that explicit bigotry was fading (and the process in which more gay and lesbian citizens come out, leading to less explicit bigotry, leading to more people coming out makes it likely that it will continue to fade). Ten years ago it wasn't at all clear to me that marriage equality would win, but I was very confident that if and where it won, public opinion against it would rapidly collapse.
But I don't think that support for weed legalization is similar. There does appear to be a generational effect, which suggests that the trend will continue to some extent. But it's not at all hard to imagine it reversing. As Mark Kleiman says, marijuana may be less harmful than alcohol, but that doesn't mean it's harmless -- and it's easy to imagine any number of weed-related problems that could make for bad headlines and reverse public opinions. I'm not predicting that, mind you; it's just that I think future trends depend on unpredictable events (or, to be more precise, unpredictable press interpretation of unpredictable events).
There's also a politician piece to that. Politicians "evolving" on marriage were only worried, really, about current opinion. I'm pretty sure they didn't worry about a backlash if marriage equality was enacted, and certainly not that if marriage equality took place that newspapers might start running gay marriage horror stories (has even the GOP-aligned press ever run gay marriage horror stories? What would they look like?).
But politicians will worry that if they support legalization that they could be held responsible for any weed horror stories that emerge -- and everyone knows that the press is capable for concocting those, true or not.
Supporters of legalization can argue that horror stories are not particularly likely to show up because (1) they're not apt to be true, and (2) given public opinion, the press won't find them appealing. That might be true! I'm only suggesting there's a lot more uncertainty here than one might think, especially if one believes in the marriage analogy.