As you're probably aware, President Obama announced today his support for allowing same-sex couples to marry. It's a move that might change the race in several ways:
-Finally there is something approximating a real basis for calling Obama a "radical". Before, all they had was that he followed a health care plan created by the Heritage Foundation and the 1990's Republican Party, that he continued and intensified many of Bush's anti-terror programs, that he advocated returning to a top tax rate which didn't dampen economic growth during the Clinton years, and that he pursued economic policies which were not at all controversial just a few years ago. But it's tough to call someone a radical for advocating a position which growing pluralities/majorities of Americans support.
-Proposition 8, which in California re-instated a ban on gay marriage, was overwhelmingly supported by African-Americans, a core Obama constituency. I don't, however, think there will be very many African-Americans who would vote to designate the first African-American President a failure, which a loss for Obama would do.
-It very well may hurt him amongst the more conservative Democrats and independents. As shown by the recent vote in North Carolina, gay marriage is not popular in several swing states.
-Just about anything that distracts from discussions of the economy is good for Obama.
-Most importantly, opposition to gay marriage is not a position which stands up to scrutiny, as discussed yesterday. By elevating the issue as he did today, Obama will force a more detailed discussion of the matter. What could Romney possibly say in such a discussion? That it says gay sex is bad in Leviticus? By provoking a real discussion of the issue, Obama will expose that there is nothing approximating a rational basis for the conservative position. Given Romney's tenuous standing with evangelicals, it's a fight that he can't avoid. If he fails to attack Obama for his position, he'll endanger his standing with his own base.
So while the numbers might currently suggest that Obama's "evolution" on the issue might hurt him in swing states, the current numbers don't matter. 6 months of discussion in which one side is utterly without merit will likely change those numbers.