President Obama’s decision to express support for gay marriage in an interview with ABC’s Robin Roberts is a decision that comes with real political risk.

View Photo Gallery: President Obama endorsed same-sex marriage Wednesday, making him the latest — and most prominent — U.S. politician to call for gay nuptials. Here are a few other prominent politicians who have supported gay marriage.

In truth, Obama’s hand was forced on the issue by comments made by Vice President Joe Biden on NBC’s “Meet the Press”in which he seemed to express support for gay marriage.

(Watch the video: Obama supports gay marriage )

While Biden’s office and the White House insisted there was no space between Biden’s remarks and Obama’s stated position in support of civil unions but not gay marriage, it reeked of bad spin — and everyone (including the White House) sensed it.

Given that tenuous situation and the fact that it was something close to an accepted fact in political circles that Obama, on a personal level, favored allowing gays to marry, it was clear that the President had to say something — and sooner rather than later.

Now that the President is on record in support of gay marriage, it’s worth breaking down the political pluses and minuses of that position.

Let’s start with the pluses.

1. Public opinion: The movement in national polling on the issue is something rarely seen; support for gay marriage is rapidly rising even as opposition rapidly erodes.

Here’s a chart of the legal/illegal question in Washington Post-ABC News polling since 2004:

Those trend lines suggest that large majorities of the public will be supportive of the legalization of gay marriage before too long.

2. Excite the activist base: The LGBT community is not only a major part of the Democratic base but they also comprise some of the most politicalyl active people in the party. While Obama had a case to make — his decision to end “Don’t Ask, Don’t Tell” and the Justice Department’s announcement that they won’t defend the constitutionality of the Defense of Marriage Act — that he had already done a lot for the LGBT community, this is a cherry on top that wins him not only their votes but, as importantly, their volunteer hours.

3. Excite the donor base: One in six Obama’s 2012 campaign bundlers are gay, according to research conducted by the Post’s Dan Eggen and T.W. Farnam. Couple that fact with the news earlier this week that George Soros, one of the most prominent Democratic major donors in the country, was directing his contributions to grassroots groups rather than the super PAC expected to fund TV ads supporting Obama and it’s clear that the president badly needs a heavy investment from Hollywood and Silicon Valley. And those two communities are heavily in favor of gay marriage. (Don’t forget: Obama is headed to Hollywood tonight for a fundraiser at the house of actor George Clooney.)

4. Rally young people: It’s no secret that for President Obama to win, he needs a heavily consolidated vote among 18-29 year olds. And, polling suggests that the age group most supportive of gay marriage is — you guessed it! — 18-29 year olds. In a year’s worth of Post-ABC polling — from March 2011 to March 2012 — fully 65 percent of people aged 18-29 say gay marriage should be legal while 61 percent of those aged 30-39 said the same.

5. A principled stand: Since Biden’s comments on Sunday, the White House — and the President — have been thrust into an awkward position. If he is still evolving on the issue, why? What other information does he need? He has looked like exactly what his team doesn’t want him to be: a regular old politician. Watch for the campaign to cast his decision to come out in support of gay marriage as just the sort of principled stand that differentiate him from that image.

And now, the minuses.

1. Not a slam dunk: As we wrote earlier this week, while a majority of people in the country now favor gay marriage it’s far from the sort of overwhelming majority that would prompt Obama to change his position without, of course, the (unplanned) prompting of Biden. Key subgroups like white voters (53 percent support legalization/43 percent oppose it) and voters aged 40-49 (52 percent support) are only narrowly on board with legalization. And, in an election year, no politician — including Obama — wants to tackle an issue where public opinion is that closely divided.

2. The black community: If the LGBT vote is a pillar of Obama’s base, the African American vote is an even bigger pillar. (In a recent Post poll in Virginia, Obama was beating former Massachusetts governor Mitt Romney 97 percent to one percent among black voters.) And, African Americans have consistently been one of the groups most resistant to gay marriage. In that year’s worth of Post-ABC polling, just 42 percent said they support legalization while 55 percent oppose it. Yes, it’s unlikely that black voters leave Obama in droves over the issue. But it’s easier to see this decision taking some of the passion for him out of this community.

3. Virginia/North Carolina: President Obama won both states in 2008 and his campaign is targeting both again in 2012. (The Democratic National Convention this summer will be held in Charlotte.) While both states have changed demographically and politically over the past decade (or so), large portions of both states remain culturally quite conservative and might balk at Obama’s support for same-sex marriage. (Amendment One, which makes the only legal union in the state as between a man and a woman, passed with 60 percent of the vote on Tuesday in North Carolina.)

4. Flip flopping: Romney has a reputation for flip flopping that Democrats have made considerable political hay out of over these past few months. Obama’s change on gay marriage allows Republicans a strong counter to the charge that their nominee is the only one in the race who lacks a core. While it’s hard to imagine Republicans devoting all that much time to a social issue — and therefore away from the economy — it gives them a way to fight back on a perception that clearly is hurting Romney.

In the end, President Obama had little choice but to get off the fence on the issue given the furor caused by Biden. The decision is now made. How it plays politically won’t be determined for several months — or maybe until November.

Watch a clip from Obama’s interview below:

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