I wrote a piece that you can find elsewhere on the Beast—or, lo and behold, simply by clicking here—about how Obama can turn his big announcement into a political plus. Money graf:
So this is what Obama needs to do: When the subject turns to this issue, he needs to make sure that Americans know that Romney opposes even civil unions, and that he would seek to outlaw gay marriage across the country, and he needs to make Romney defend those positions. Obama, in contrast, can say: “Hey, look, I took a personal position. I’m not trying to make Alabama or Oklahoma do anything they don’t want to do. But you, sir, would take already-won rights away from gay couples whose unions are now recognized in a number of states.” And then he drops this bomb: “My position is no different from Dick Cheney’s. Is he outside the mainstream?” And then, after that, he can say something like: “Governor, your own father bravely broke from his party on a great civil-rights matter. Why can’t you?”
That's about the size of it, but obviously you need to read the entire thing. I'm not entirely persuaded this will work out for him politically, but the column is my best guess as to how it might.