WASHINGTON: US President Barack Obama took a calculated gamble and stepped into the political unknown Wednesday with his firm public backing for gay marriage, after a long period of personal soul searching.

Obama's move, in an interview with ABC News, sent seismic waves through pre-election politics and sparked immediate speculation as to whether he had hindered his chances of winning a second term in November.

But it also led his election foe Mitt Romney and his Republicans onto tricky ground, as the party's social conservative base opposes same sex marriage, even as it becomes quickly more accepted across the broader political spectrum.

“I've just concluded, for me, personally, it is important for me to go ahead and affirm that I think same-sex couples should be able to get married,” Obama said, unveiling his bombshell.

Obama has now abandoned the sheltered but increasingly untenable position that he was “evolving” on gay marriage.

The risks are clear, and dictated Obama's previous stance, despite fierce pressure for a more unequivocal stance from Obama's liberal base.

The sudden injection of a divisive moral-social question could hit Obama's prospects in battleground states that he needs in November.

He has always had trouble connecting with white, blue collar, socially conservative swing voters, and Wednesday's move will hardly help.

A case in point is North Carolina, which Obama won by less than one percent in 2012. On Tuesday 61 percent in the state voted to ban gay marriage, civil unions and domestic partnerships.

Some observers feel Obama could face a backlash from religious Hispanic and African-American voters, who helped sweep him into the White House in 2008.

“There may be a few white, religious Democrats whom he could lose in Ohio,”said Professor Paul Beck of Ohio State University, adding that some Hispanics could drift away in Colorado or Florida.

“Still, I think the numbers will be small who are swayed at all by the position he took today,” he said, ahead of an election shaping up to be dominated by the lagging economy.

Conservatives pledged to make Obama pay.

“Today's announcement almost ensures that marriage will again be a major issue in the presidential election,” said Tony Perkins of the Family Research Council.

A senior Obama aide said it was unclear how the marriage issue would play politically, especially in battleground states such as Ohio. Democrats shudder when they recall how ballot initiatives in swing states on social issues drove up Republican turnout in 2004 and helped defeat their presidential candidate John Kerry. As another senior official explained, public perceptions on gay marriage are changing more quickly than almost any other political issue in America.

More people are coming into contact with families with single sex parents — a point Obama made in his interview when he said that he had talked about the issue with his girls, Malia and Sasha.

A Washington Post poll suggests that the US public has, like Obama, been evolving.

In 2006, only 36 percent backed legalizing same-sex marriage, but in the latest poll, 52 percent said they were for it.

The decision was greeted with joy in the gay and lesbian community and is certain to fire up Obama's political base, and give younger supporters seen as less energized than in 2008 a jolt.

Wealthy gay and lesbian voters are also important contributors to Obama's reelection campaign.

One more question about Obama's move on Wednesday was: Why now?

He seems to have been pushed by Vice President Joe Biden, who said on NBC on Sunday that he was “absolutely comfortable” with same-sex marriage.

A senior Obama official said that though Biden's move had advanced the process, the president had already concluded that he had to come out for gay marriage before the Democratic National Convention in September.

The official also used the issue to draw a sharp contrast with Romney, who the Obama camp has been portraying as extreme in a bid to limit his appeal to the key political middle.

He noted that Obama now favored gay marriage while Romney wanted to enshrine discrimination with a federal amendment to the US Constitution to ban it.