WASHINGTON - President Barack Obama declared his unequivocal support for gay marriage on Wednesday, a historic announcement that gave the polarizing issue a more prominent role in the 2012 presidential race.
The announcement was the first by a sitting president, and Republican challenger Mitt Romney swiftly disagreed with it.
"I believe that marriage is between a man and a woman," Romney said while campaigning in Oklahoma.
Gay rights advocates cheered Obama's declaration, which they had long urged him to make.
Obama revealed his decision after a series of events that made clear the political ground was shifting. He once opposed gay marriage but more recently had said his views were "evolving."
In an ABC interview in which he blended the personal and the presidential, Obama said that "it wouldn't dawn" on his daughters, Sasha and Malia, that some of their friends' parents would be treated differently than others.
Obama said he also thought of aides "who are in incredibly committed monogamous same-sex relationships who are raising kids together."
Obama added that he thought about "those soldiers or airmen or Marines or sailors who are out there fighting on my behalf, and yet feel constrained even though now that 'don't ask, don't tell' is gone because they're not able to commit themselves in a marriage."
Nevada's two U.S. senators split along party lines.
Democratic Sen. Harry Reid, the Senate majority leader, said he personally believes that marriage is between a man and a woman. "But in a civil society, I believe that people should be able to marry whomever they want, and it's no business of mine if two men or two women want to get married."
"The idea that allowing two loving, committed people to marry would have any impact on my life, or on my family's life, always struck me as absurd," Reid said in a statement. "In talking with my children and grandchildren, it has become clear to me they take marriage equality as a given. I have no doubt that their view will carry the future."
Reid also said it's an issue that should be determined by state law.
Sen. Dean Heller, R-Nev., repeated his long-standing opposition to gay marriage. "I believe that marriage is between one man and one woman and would not support changing that," he said in a statement.
Rep. Shelley Berkley, D-Nev., supported Obama's stance, saying in a statement, "I have been a longtime supporter of marriage equality for all Americans and am glad President Obama has embraced this civil right."
During Wednesday's interview, Obama said he was taking a personal position. Aides said that the president's shift would have no effect on current policies and that he continues to believe that marriage is an issue best decided by states.
"I have hesitated on gay marriage in part because I thought that civil unions would be sufficient," Obama said.
He added, "I was sensitive to the fact that for a lot of people the word 'marriage' was something that evokes very powerful traditions, religious beliefs and so forth."
Now, he said, "it is important for me personally to go ahead and affirm that same-sex couples should be able to get married."
Vice President Joe Biden said on Sunday that he is completely comfortable with gays marrying . Gay rights groups cited Biden's comments in urging Obama to announce his support.
On Tuesday, voters in North Carolina, which will host the Democratic National Convention from Sept. 3-6 in Charlotte, approved an amendment to the state constitution affirming that marriage may only be a union of a man and a woman.
As recently as eight years ago, conservatives in several states maneuvered successfully to place questions relating to gay marriage on the Election Day ballot as a way of boosting turnout for President George W. Bush's re-election.
Now, nationwide polling suggests increasing acceptance of gay marriage. In a survey released this month, Gallup reported 50 percent of those polled said it should be legal, and 48 percent were opposed. Democrats favored by a margin of 2-1, while Republicans opposed it by an even bigger margin. Among independents, 57 percent expressed support, and 40 percent were opposed.
Shifting his emphasis could open Obama up to criticism that he is taking his eye off the economy, voters' No. 1 issue. Yet some gay donors have said they wanted Obama to announce his support for gay marriage. Other Democratic supporters contend Obama's decision could energize huge swaths of the party, including young people. He also could appeal to independent voters.
By day's end Wednesday, the Obama campaign had emailed a clip of the interview and a personal statement from the president to supporters.
Obama said he sometimes talks with college Republicans , and while they oppose his policies on the economy and foreign policy, "when it comes to same sex equality, or, you know, sexual orientation, that they believe in equality. They are more comfortable with it."
Maggie Gallagher, co-founder of the National Organization for Marriage and a leading supporter of the constitutional amendment approved in North Carolina on Tuesday, said, "Politically, we welcome this. We think it's a huge mistake. President Obama is choosing the money over the voters the day after 61 percent of North Carolinians in a key swing state demonstrated they oppose gay marriage."
Acknowledging his support for same-sex marriage may rankle religious conservatives, Obama said he thinks about his faith in part through the prism of the Golden Rule, treating others the way you would want to be treated.
"That's what we try to impart to our kids and that's what motivates me as president and I figure the most consistent I can be in being true to those precepts, the better I'll be as a dad and a husband and hopefully the better I'll be as president," Obama said.
Six states and the District of Columbia allow same-sex marriages. In addition, two other states have laws that are not yet in effect and may be subject to referendums.
The Las Vegas Review-Journal contributed to this report.