Now look at the latest Quinnipiac poll in Virginia, released this week. Against Mitt Romney, Obama wins only 36 percent of white voters in the state -- a weak showing. But thanks to 95 percent support among black voters, Obama leads in the general election matchup, 50 to 42 percent. But that's dependent on minority turnout being close to the ideal Obama scenario; any leveling off would make it more challenging for the president to carry the state again.

And that's where the gay marriage debate gets awfully tricky for the president's political advisers.

Public opinion on gay marriage is pretty straightforward. There's a huge generational divide -- with older voters solidly opposed, and younger voters solidly supportive. Over time, support for gay marriage should increase. And if Obama came out in support of gay marriage, he would probably excite and inspire many college-aged Democrats to show up at the polls and support him -- not to mention winning back some socially-moderate independents who have been disenchanted with the president over the economy.

But college-aged voters are only part of the president's coalition. The bigger element consists of African-American voters, who are solidly opposed to gay marriage. California's Proposition 8 ban on same-sex marriage passed in 2008 thanks to overwhelming black support; 70 percent backed it, according to exit polls. Recent gay marriage legislation in Maryland drew opposition from leading Democratic African-American legislators in the state. The same ministers organizing get-out-the-vote efforts in black churches for Obama are also railing against gay marriage.

Obama can't afford to even risk losing the deep enthusiasm black voters have towards him. They gave Obama a whopping 95 percent of the vote against John McCain last year and turned out at historic levels. He should get similar levels of support this year, but with the down economy disproportionately affecting the black community, he's not at all assured that they'll turn out at the same level as 2008. Backing gay marriage would virtually guarantee that some would stay home -- perhaps enough to tip the balance in states like Virginia, Pennsylvania, North Carolina, and Ohio.

The conventional wisdom has been that supporting gay marriage would alienate blue-collar whites, and that's been the main reason he's been hesitant to come out in favor before the general election. But in this case, it's a crucial element of his own base that's preventing the president from taking bolder steps to advance a cause that he seems to believe in, but hasn't publicly embraced. It's as much about politics as principle.

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