WASHINGTON — Gay marriage is poised to play a pivotal role in American politics this year with President Barack Obama becoming the first sitting president to endorse same-sex marriage.

As recently as Monday, White House officials had said Obama’s position on gay marriage was still in flux. But in an interview at the White House with ABC News, Obama cited his own staffers in “incredibly committed” same-sex relationships, as well as openly gay service members, as factors in making up his mind.

“At a certain point I’ve just concluded that 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," he told ABC News’ Robin Roberts.

Obama once opposed gay marriage but has said for two years that his position was “evolving.” His election-year conversion comes amid growing pressure from his own party and close advisers to endorse gay marriage, with some Democrats pressing to include the issue in the party’s national platform.

Republicans decried it as a political conversion, but Obama said he believes opinion is changing in the U.S. – even though the interview came a day after voters in North Carolina approved a state constitutional amendment barring same-sex marriage.

Obama said he believes the opposition is “generational,” noting that he’s talked with college Republicans “who think that I have terrible policies on the economy, on foreign policy, but are very clear that when it comes to same-sex equality . . . they believe in equality. They are much more comfortable with it.”

And he said his young daughters, Sasha and Malia, have friends whose parents are same-sex couples.

"It wouldn’t dawn on them that somehow their friends’ parents would be treated differently," Obama said of his daughters. "It doesn’t make sense to them and frankly, that’s the kind of thing that prompts a change in perspective."

The president’s decision to embrace gay marriage comes just six months before the election and carries both risk and benefit for Obama, who has courted gay voters but frustrated many of them with his reluctance to endorse gay marriage.

Democratic National Committee treasurer and gay activist Andrew Tobias said the decision had “the little credit card machine in my head… running overtime.”

“People are looking for a way to express their enthusiasm – and to help make sure this great president gets a second term to finish so much of the crucial stuff, LGBT and otherwise, he’s begun,” Tobias said.

Conservatives assailed Obama’s decision as political and vowed to defeat him at the polls.

Hours before the president’s announcement, presumptive Republican presidential nominee Mitt Romney restated his opposition to same-sex marriage in an interview with KDVR-TV in Colorado.

"When these issues were raised in my state of Massachusetts, I indicated my view, which is I do not favor marriage between people of the same gender, and I do not favor civil unions if they are identical to marriage other than by name," the former Massachusetts governor said. "My view is the domestic partnership benefits, hospital visitation rights and the like are appropriate, but that the others are not."

Gay-rights activists and some Democratic Party officials have been pressing the case for months, and activists hailed the decision as historic.