“He now becomes the first sitting president to join the majority of Americans whose hearts have opened and minds have changed in favor of the freedom to marry,” said Evan Wolfson, the founder of Freedom to Marry, who called Obama’s support a “historic turning point for the freedom to marry movement.”

New York City Mayor Michael Bloomberg called it a “major turning point in the history of American civil rights.”

“No American president has ever supported a major expansion of civil rights that has not ultimately been adopted by the American people,” Bloomberg said. “And I have no doubt that this will be no exception.”

The pressure on Obama spiked over the weekend when two of Obama’s closest advisers – Vice President Joe Biden and Education Secretary Arne Duncan – both expressed personal support for same-sex marriage.

Though public opinion polls show support for same-sex marriage steadily increasing over the past five years, 44 states don’t allow gay couples to marry.

Yet Americans have grown more accepting of same-sex marriage over the last decade. Not broadly supportive, but more open to it. The Pew Research Center, for example, found 47 percent of Americans favoring legal same-sex marriage and 43 percent opposing. In 2008, 38 percent favored it and 49 percent were opposed.

Polls show an age divide: A March survey by the Public Religion Research Institute found 52 percent of Americans favoring gay and lesbian marriage and 44 percent opposed. Of those between the ages of 18 and 29, 74 percent backed gay marriage, compared with 33 percent of people 65 and over.

The National Organization for Marriage vowed to defeat Obama in swing states such as Ohio, North Carolina, Virginia, Florida and Nevada, where voters have adopted constitutional amendments that bar gay marriage.

“President Obama has now made the definition of marriage a defining issue in the presidential contest,” said the organization’s president, Brian Brown, adding that his group would work “ceaselessly” to “preserve traditional marriage.”

“God is the author of marriage, and we will not let an activist politician like Barack Obama who is beholden to gay-marriage activists for campaign financing to turn marriage into something political that can be redefined according to presidential whim,” Brown said.

Obama has long wrestled with the subject, flip-flopping over the years. He seemingly first supported it, then was against it, and on Wednesday, he came out for it again.

"I favor legalizing same-sex marriages," he said in a statement while running for the Illinois state Senate in 1996. Aides later said the statement did not reflect his stance.

In 2004, while running for the U.S. Senate, Obama said that he believed that “marriage is between a man and a woman. What I believe, in my faith, is that a man and a woman, when they get married, are performing something before God, and it’s not simply the two persons who are meeting."

"Personally, I do believe that marriage is between a man and a woman,” he said in a 2007 Senate debate.

But he also started signaling that he could change his mind – though he maintained opposition to gay marriage during the 2008 presidential campaign.

He wrote in his 2006 book, “The Audacity of Hope,” that "It is my obligation, not only as an elected official in a pluralistic society but also as a Christian, to remain open to the possibility that my unwillingness to support gay marriage is misguided, and that in years hence I may be seen as someone who was on the wrong side of history.”

"My feelings about this are constantly evolving," he said in 2010. "I struggle with this."

In Wednesday’s interview, Obama said he had been “going through an evolution on this issue.” He said he had “always been adamant that gays and lesbians should be treated fairly and equally” but had “hesitated” on gay marriage because he believed civil unions were adequate in protecting their rights.

“And I was sensitive to the fact that for a lot of people, the word marriage was something that evokes very powerful traditions, religious beliefs,” he said.

But Obama said that he and first lady Michelle Obama have discussed the issue and that she “feels the same way that I do.”

“The values that I care most deeply about and she cares most deeply about (are) how we treat other people,” he said. “We are both practicing Christians and obviously this position may be considered to put us at odds with the views of others, but when we think about our faith, the thing at root that we think about is, not only Christ sacrificing himself on our behalf, but it’s also the Golden Rule, treat others the way you would want to be treated.”

Miami Herald staff writer Steve Rothaus contributed to this report from Miami.