OK, the interview on ABC has Dear Leader finally following his senior citizen vice president, and the ideas of some young buck named Barack Obama in 1998 and supporting gay marriage.(Apparently the NYT's The Caucus blog hasn't heard of Obama 1998, saying this is the first time he's supported gay marriage. Probably another reason the paper of record doesn't want a real paywall, if it's going to make errors like that in national political coverage.)
Polls show a small majority favor it, but I've not seen detailed breakouts of independent voters.
By age, gay marriage is greater among younger people, but they turn out to vote in lower numbers.
Since we've seen North Carolina's vote on the issue, I think it's safe to say it hurts there, and likely Indiana.
I'll call it a bump in Virginia and Colorado, a wash in Nevada, and unknown right now in Ohio.
That said, since Mitt Romney is now "no comment" on the subject, it behooves Obama surrogates to push him on the issue.