This is a very unpredictable move on Obama's part, in terms of political repercussions. Very uncharacteristic of his usually methodological and reserved approach.
This could be great, and give him a big advantage because the left will see the election as a referendum on gay marriage and be way more motivated with donations (many will probably consider an Obama loss something that will set the gay-rights movement back a decade). It also has a chance of getting the GOP very off message when they should be focusing on the economy.
On the other hand, this could hypothetically lose Obama a lot of support among swing voters (especially in more working-class states). Not to mention that the religious opposition will be extreme.
If this happens Obama is done for. Romney shall accuse him of trying to shift the election's focus from the economy to try to take off the spotlight from his own failures, whether real or not.
But that's the beauty of this. Obama doesn't have to mention anything else about gay marriage for the rest of the year. This single signal is enough to get equality advocates seriously motivated. He can and certainly will continue to stay on message arguing that his policies saved the economy, things are getting better, etc., however his campaign is phrasing it.
Obama has said, "yeah, I support gay marriage," and that's all he has to do. Now Romney can either ignore it and stay on the economy (but miss opportunity to win over religious swing voters) or respond and risk losing his campaign's narrative. Certainly a dilemma for Romney's people, I'd imagine.
