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

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

But it also led his election foe Mitt Romney and his Republicans on to 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 to go ahead and affirm that I think same-sex couples should be able to get married,” Mr Obama said, unveiling his bombshell.

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

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

The sudden injection of a divisive moral-social question could hit Mr 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 Mr Obama won by less than one per cent in 2012. On Tuesday, 61 per cent in the state voted to ban gay marriage, civil unions and domestic partnerships.

Some observers feel Mr 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 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 Mr 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.

Dennis Goldford, a professor of political science at Drake University, Iowa, suggested Mr Obama’s decision reflected an “electoral and strategic calculation”.

Democrats are more likely to favour gay rights, and those who most oppose the concept tend to be conservatives who would never vote for Mr Obama anyway, he said.

“The question is whether there is anybody in the muddled middle on this issue, that this might alienate. But we don’t know at this point.”

It is just possible that Mr Obama has pulled off a political master stroke, and decided to ride changing attitudes just at the right moment.

As another senior official explained, public perceptions on gay marriage are changing more quickly than almost any other political issue in the US.

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

A Washington Post poll suggests that the US public has, like Mr Obama, been evolving. In 2006, only 36 per cent backed legalising same-sex marriage, but in the latest poll, 52 per cent said they were for it.

The decision was greeted with joy in the gay and lesbian community and is certain to fire up Mr Obama’s political base, and give younger supporters – seen as less energised than in 2008 – a jolt. Wealthy gay and lesbian voters are also important contributors to Mr Obama’s re-election campaign. But one more question about this move was: Why now?

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

A senior Obama official said that though Mr 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 Mr Romney, whom the Obama camp has been portraying as extreme in abid to limit his appeal to the key political middle.

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

Election facts

• President Barack Obama is running for a second and final term in office.

• His main challenger is former Massachusetts Governor Mitt Romney.

• Mr Obama’s campaign focuses on eliminating tax incentives for companies that send jobs overseas, measures to make it easier to refinance home loans, tax credits for firms that create new jobs or raise pay, expanded tax credits for investments in clean energy and help veterans returning from war to get jobs.

• This is the first US presidential re-election campaign to use Twitter and Facebook for promotion.