Gallup took a poll of adults regarding Obama's endorsement of gay marriage, and the topline number seems* to look good for the president: 51% approve of Obama's support of same-sex marriage, while 45% disapprove. But when you take a look at how it might move votes, it seems to be a losing issue for Obama: 13% of Americans say Obama's endorsement of gay marriage makes them more likely to vote for Obama, while 26% of Americans say the move makes them less likely to vote for Obama.
*The topline number should be taken with a large grain of salt. As Ross Douthat notes, research by NYU political scientist Patrick Egan shows that "the share of voters in pre-election surveys saying that they will vote to ban same-sex marriage is typically seven percentage points lower than the actual vote on election day."
As the pollster for the Democratic firm PPP wrote Tuesday night, "Hate to say it, but I don’t believe polls showing support for gay marriage nationally. Any times there is a vote it doesn’t back it up."