Not according to Jonathan Bernstein:
At the margins, Obama’s new position makes him slightly more prepared for the fall campaign; should marriage hit the headlines (say, from a major court decision), it’s probably easier for him to talk about it now than it was under his old position. And I'm not saying that social issues overall are not important politically. They certainly can be — but their importance, so to speak, has already happened; those who care about these things have sorted themselves by party long ago. So mostly, as far as November is concerned, this new wrinkle just doesn’t seem likely to have any effect at all.
Electionate agrees:
Obama’s coalition is far more resilient to opposition to gay marriage than any Democrat in history. If Obama was trying to reassemble the old Democratic coalition, which required Democrats to win a near majority of the white working class vote to win the Presidency, gay marriage would be a terrible proposition for Democrats electoral chances, since winning half of the non-college white vote would require winning many opponents of gay marriage. Today, Obama would be thrilled with 40% of the non-college white vote, and can probably win beneath 35%. This more modest feat will require winning far fewer conservative, white gay marriage opponents. Similarly, gay marriage would be far more threatening to a Democrat who couldn’t count on historic levels of support and enthusiasm from African American voters.
Amanda Marcotte argues along the same lines:
With the gay rights issue, the don't-care factor is rising rapidly alongside the open support for same-sex marriage. Which is to say that a lot of people who oppose gay marriage do so in a softer way than before. They will hold on to their belief, but they also see the writing on the wall and are adjusting their commitment to this issue accordingly. They'll vote against gay marriage in a special election, sure, but they've also decided they're not going to lose any sleep if the courts declare same-sex marriage a right.
Another good indicator: Intrade hardly budged.
The Dish 