The political impact of President Obama’s decision to embrace gay marriage is almost impossible to predict, as the issue cuts along racial, generational and cultural lines in a way few subjects do.

Obama is certainly taking some political risk, as voters in 30 states, including several he wants to win this fall, have passed gay marriage bans. But polls a growing number of Americans, more than half in some surveys, back gay marriage. Top Republican officials, aware of the complexity of the issue, did not leap to attack Obama, as they do on most matters.

theGrio: Gay-marriage foes sought to split gays and blacks

Former Massachusetts Gov. Mitt Romney yesterday appeared uncomfortable with the subject, repeating his view marriage is between a man and a woman but adding this is a “very tender and sensitive topic, as are many social issues.” He seems unlikely to make it a major subject of emphasis in his campaign.

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Polls have shown Republicans, evangelical Christians, voters over the age of 65 and African-Americans in particular are more likely to oppose gay marriage than other voting blocs. But Republicans and evangelical Christians were already going to vote against Obama, and it’s unlikely a difference on one issue will meaningfully weaken Obama’s ironclad support in the black community.

Liberal and gay rights advocates have cheered Obama in the last 24 hours, but he already had their votes anyway. Buzzfeed has reported Obama raised $1 million for his campaign after news broke about his gay marriage stance, and while that kind of energy from his base won’t hurt, the two sides and their allies are expected to spend more than $1 billion this year on the election.

Here are the two groups worth watching. First, polls have suggested voters ages 18-30 are still pro-Obama, but less enthusiastic than in 2008, and perhaps not as eager to turn out. Does the gay marriage decision shift this very pro-marriage group into feeling more excited about Obama overall and therefore more likely to vote for him in large numbers like four years?

Conversely, Obama was likely to lose the over 65 vote anyway. (He lost it by eight points in 2008) But does he lose elderly voters in key states like Ohio and Florida who agree with him on economic issues but think his gay marriage stand goes too far?

Follow Perry Bacon Jr. on Twitter at @perrybaconjr