Josh Green finds some evidence that marijuana propositions help Democrats:
I found a few Democratic consultants who have become convinced that ballot initiatives legalizing marijuana, like the one Californians will vote on in November, actually help Democrats in the same way that gay marriage bans were supposed to have helped Republicans. They are similarly popular, with medical marijuana having passed in 14 states (and the District of Columbia) where it has appeared on the ballot. In a recent poll, 56 percent of Californians said they favor the upcoming initiative to legalize and tax pot. The idea that this helps Democrats is based on the demographic profile of who shows up to vote for marijuana initiatives--and wouldn't show up otherwise.