Waldman compares Democrats’ support of marriage equality to their support of marijuana legalization:
I’m not saying that by the end of the 2016 race every Democrat will support legalization of marijuana for recreational purposes. But what the experience of same-sex marriage tells us is that when opinions about an issue are changing, politicians change too. And that means that where a candidate stands right now may or may not tell us where he or she will stand two years from now. So my guess is that all the Democrats will support some kind of legalization. The more tentative ones may support an extremely limited medical legalization of the kind that now exists in the District of Columbia, where there are only a few approved conditions (and a grand total of 120 people have signed up). Others may favor a looser medical legalization that is still limited in some way. But I’ll bet some Democratic candidate, maybe one who’s “serious” but is still an underdog and thus has little to lose, will come out in favor of legalization.
(Chart from (pdf) Pew.)