Less than 24 hours after the US state of North Carolina voted on Tuesday to amend its constitution to prohibit marriage of gay couples, US President Barack Obama made history by coming out in support of same-sex marriage.

In a US television interview, Obama said, "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".

The president's position is an election year gamble. Obama's unequivocal support for gay marriage may be historic, but with a US presidential election just six months off, it comes with significant political risk.

Already, while campaigning in Oklahoma on Wednesday, Obama's Republican challenger Mitt Romney publicly disagreed with the president telling reporters he believes, "marriage is between a man and a woman".

He indicated he would support a federal constitutional amendment defining marriage as only between a man and a woman, as the southern state of North Carolina did with overwhelming support.

Americans divided

The gay marriage debate has divided Americans for years. Within hours of the president's declaration in support for gay marriage, the issue became a rallying point for Republicans who mostly oppose Obama’s views.

While on assignment in North Carolina, I heard numerous talk radio stations filled with calls from listeners who claimed Obama's support for gay marriage is the issue that finally triggered them to unite behind the presumptive Republican nominee.

Christian conservatives told me the president's position will alienate voters. They said it indicates Obama has taken his focus off the struggling US economy and jobs market, which for most Americans, remains a number one priority.

It's a calculation that could prove costly for Democrats hoping to attract undecided Conservatives that had, until now, been ambivalent toward Romney.

Another threat to Obama's re-election chances could be the reaction of African-American voters to his same-sex marriage stand.

The voting bloc has long been among Mr. Obama's most faithful supporters, but they are also some of the most churchgoing groups in America.

Openly hostile

Those I spoke with were openly hostile to the notion of gay marriage and said it violated their personal belief that marriage is a religious contract defined by a biblical definition of a union only between a man and a woman.

They bristled at suggestions the march toward equal rights for same-sex couples was similar to the civil rights struggle in the 1960s, when African Americans in the United States also fought for equality.

Polls in the US show Americans overall remain divided on the issue of gay marriage, but steadily are moving towards acceptance.

Six states plus the District of Columbia now allow same-sex marriages, and despite the vote in North Carolina, more seem poised to follow.

A nationwide poll earlier this year showed 50 per cent of respondents felt gay marriage should be legal, and 48 percent were opposed.

Young people seem most in support.

This is a plus for Obama's re-election efforts.

His campaign has been struggling to recapture the excitement of his 2008 effort, especially among his young supporters, who felt his promises of four years ago, fell flat.

There was concern those voters would be absent on election day. Obama's support for same-sex marriage could provide a boost with that demographics' turnout.

Still, from a political standpoint, the president's endorsement of gay marriage is a risky stand.

Risk calculation

While it's sure to garner praise, and cash, from liberal supporters, it will almost certainly cost him in terms of support in more conservative southern states.

Regardless, the president's advisers seemed to have determined it's a calculation worth the risk.

By declaring his support for gay marriage, President Obama has created, for the first time, a clear contrast between himself and his political rival, Mitt Romney.

The stand for, or against gay marriage is likely to energise large numbers of voters who had up until now, been less than enthused about the upcoming November presidential vote.

The question now is whether Obama's support for same-sex marriage will attract more voters than it drives off. It is a political risk the President now seems willing to take.