President Barack Obama said Wednesday he supported gay marriage, reversing his position on a controversial social issue just six months before the November election and adopting a stance fraught with uncertain political implications.

Mr. Obama had been under intense pressure this week to lay out a clear stance on same-sex marriage after Vice President Joe Biden and other top advisers endorsed it. Mr. Obama said that after years of lengthy discussions with friends and family, including his wife and two young daughters, he now "personally" believes gays and lesbians should have the right to marry.

"I've been going through an evolution on this issue. I've always been adamant that gay and lesbian Americans should be treated fairly and equally," Mr. Obama said in a television interview with ABC. "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."

Mr. Obama is the first sitting U.S. president to publicly support gay marriage. His endorsement is largely a symbolic moment for a country that is actively wrestling with the issue.

While the president opposes the federal Defense of Marriage law that defines marriage as a union of a man and a woman, he doesn't plan to pursue new U.S. policy on gay marriage, aides said, because he believes states should decide the issue.

Mr. Obama was against same-sex marriage as a candidate in 2008 but supported civil unions. In the fall of 2010, he said his views on gay marriage were "evolving," a stance widely interpreted as moving toward an endorsement. Asked numerous times afterward whether his position had changed, the president deflected the question and pointed to his record on other gay-rights issues.

On Wednesday, he explained, "I had hesitated on gay marriage, in part because I thought civil unions would be sufficient… And I was sensitive to the fact that for a lot of people the word 'marriage' was something that invokes very powerful traditions, religious beliefs and so forth."

He said his position was influenced by gay members of the military and his staff who are raising children together in monogamous relationships.

Mr. Obama informed a handful of top aides earlier this year he had decided to publicly support gay marriage before the Democratic National Convention in September, senior administration officials said. Mr. Biden's public support, in a TV interview Sunday, and the 72 hours of fallout that came afterward sped up the president's timetable, these officials said.

President Obama spoke Wednesday with Robin Roberts of ABC's Good Morning America in the Cabinet Room of the White House.

Mr. Obama's position puts him squarely at odds with that of Mitt Romney, the presumptive Republican nominee. Mr. Romney has said he believes marriage should be between a man and a woman. He also opposes civil unions and has said he would back a constitutional amendment banning gay marriage.

Asked Wednesday about Mr. Obama's gay-marriage endorsement while campaigning in Oklahoma, Mr. Romney acknowledged that it is a "very tender and sensitive topic." He said states are able to make decisions about domestic-partnership benefits, including hospital-visitation rights. "But my view is that marriage, itself, is a relationship between a man and a woman and that's my own preference," he said.

Polls show Americans' views on gay marriage are shifting faster than for many other hot-button social issues, with 49% in favor, according to a March Wall Street Journal/NBC News poll, up from 41% in 2009. Some 40% of Americans oppose gay marriage, according to the poll.

Other surveys show similar levels of backing for same sex-marriage, but some show opposition to have nearly equal support. In a recent Gallup survey, 50% approved of gay marriage, while 48% said they opposed it. But polls consistently show rising support in recent years.

Voters have enacted constitutional bans on gay marriage in a number of battleground states that will decide the 2012 election, among them Ohio and Florida. Mr. Obama's Wednesday announcement came a day after residents in North Carolina, a state the president hopes to win in November, voted overwhelmingly in favor of a constitutional amendment defining marriage as between a man and a woman. North Carolina is also hosting the Democratic National Convention, where Democrats were set to battle over whether to make gay marriage rights a plank of their party's official platform.

The issue holds potential perils for conservative Democrats, and benefits for Mr. Romney.

GOP presidential candidate Mitt Romney reacted to President Obama's endorsement of same-sex marriage Wednesday by doubling down on his own view that marriage should only be between a man and a woman.

Ralph Reed, founder and chairman of the Faith and Freedom Coalition, a social conservative group, said Mr. Obama's gay-marriage switch would fire up the GOP's conservative base. "This is an unanticipated gift to the Romney campaign. It is certain to fuel a record turnout of voters of faith to the polls this November," he said.

"Just before an election, you're going to rile up the right-wing base, there's no question about it," said Peter Fenn, a Democratic consultant. "It will hurt in rural areas and the West, and you may take some fallout in the black community."

Wading into the gay-marriage issue now poses potential risks and rewards for Mr. Obama among different types of voters who helped him win the White House in 2008.

The move could energize young voters, who support gay marriage by a wide margin—57% of 18- to 34-year-olds in the Journal poll were in favor.

But a major question is how his changed stance will be received by African-American voters, who are central to Mr. Obama's re-election strategy. Support for same-sex marriage had been relatively low among blacks, but views have evolved: The Journal poll showed African-American support for gay marriage rose to 50% in March from 32% in 2009.

Senior administration officials said Wednesday they are unsure whether Mr. Obama's new position would produce a net gain or loss in support, or whether the support and opposition for it would balance each other out.

The marriage issue is likely to resonate in future months, as White House aides said the president will continue to discuss the issue. At the same time, some Democrats are pushing for Mr. Obama to include support for gay marriage as a plank of the party platform when he accepts the nomination at the September convention. That move has the potential to divide the party.

States that have adopted constitutional bans on gay marriage also include the presidential-election battlegrounds of Wisconsin, Michigan, Virginia and Colorado. But because public opinion is fluid, it's hard to determine how Mr. Obama's view will affect his standing in those states.

Some politicians in either party appeared uneager to take up the issue Wednesday. Sen. Joe Manchin (D., W.Va.), who is running for re-election this fall, had no reaction to Mr. Obama's switch. Emily Bittner, Mr. Manchin's spokeswoman, said, "His position in the same—he believes marriage is between one man and one woman.''

Asked for his reaction, House Speaker John Boehner (R., Ohio) moved quickly to the economy. "`I have always believed that marriage is between a man and a woman, but Republicans here on Capitol Hill are focused in on economy,'' he said on the Fox Business Channel. "The American people are asking: Where are the jobs?"

Mr. Obama had essentially decided he is for gay marriage when the New York state legislature legalized it last June, aides said. Senior administration officials said the president, a former state legislator in Illinois, asked himself whether he would have voted in favor of the bill and decided he would have.

Mr. Obama's "evolving" stance had become untenable in recent days, in no small part because of the pro-gay marriage positions taken by Mr. Biden and other members of Mr. Obama's own cabinet.

Mr. Biden said Sunday on NBC that he is "absolutely comfortable" with same-sex couples getting married. Then on Monday, Education Secretary Arne Duncan, a friend of Mr. Obama's from Chicago, publicly endorsed same-sex marriage. Shaun Donovan, Mr. Obama's housing secretary, also has publicly backed gay marriage.

White House officials insisted that Mr. Biden's comments weren't intentionally meant to set the stage for Mr. Obama's announcement.

Source: The Wall Street Journal