President Obama has hinted at times that he is aware gay marriage is likely to be legal all over America in the near future, and has not disagreed with top aides, such as Secretary of Education Arne Duncan and Vice-President Joe Biden, who have said they are for it. So why is he sticking with his complicated “evolving” position, which has drawn renewed criticism after Biden’s more forthright comments this week?

The short answer may be electoral politics. Obama, as he has noted repeatedly, has done much of what a president can do on gay rights issues, from repealing “Don’t Ask, Don’t Tell” to refusing to support the federal Defense of Marriage Act, a provision President Clinton signed which defined marriage as between a man and a woman. While his declaring publicly that he supports gay marriage could shift public opinion even further toward acceptance of gay unions, marriage law is actually determined by states, not the federal government.

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But making an outright declaration of support for gay marriage carries risks. First, it would reinforce what many conservatives are saying about Obama, namely that he will shift aggressively to the political left in a second term, not having to worry about another election. It’s not exactly clear what they mean (Obama has advocated liberal goals such as universal health care strongly in his first term) but Obama taking a sharp, pro-gay marriage stand could reinforce that argument.

Second, there is a segment of swing voters, including some Catholics, who are more economically liberal (supporting higher taxes for the rich for example) but socially conservative (more supportive of abortion limits) in key states such as Ohio and Virginia. Obama is trying to cast Romney as a champion of the wealthy to win those votes. He could divert attention from his economic argument with these voters by pushing gay marriage, which some of them do not support.

Related to this, support for gay marriage divides sharply on age lines; regardless of party, younger voters are more likely to support gay marriage. The under 30 vote, of course, is already with Obama; his big challenge is older voters, particularly those over 65, who helped Republicans win the House in 2010. Those voters, some of whom are Democrats, are not yet ready to back gay marriage. (A recent ABC News/Washington Post poll showed 54 percent of adults over 65 oppose gay marriage; 61 percent of adults under 40 support it)

Some analysts have speculated that black voters, many of whom are also more conservative on civil rights issues, would defect from Obama if he supported gay marriage, but it’s not clear that would be true. The president has intense popularity in the black community.

The other element of the politics of this issue is this: would Obama gain any votes by taking a pro-gay marriage stand? Most gay rights activists will surely back the president no matter what over Mitt Romney, who has supported gay marriage bans. It’s not clear voters under 30 would turn out in higher numbers if Obama supported gay marriage more openly.

Follow Perry Bacon Jr. on Twitter at @perrybaconjr