President Obama today announced that he now supports same-sex marriage, reversing his longstanding opposition amid growing pressure from the Democratic base and even his own vice president.

In an interview with ABC News' Robin Roberts, the president described his thought process as an " evolution " that led him to this decision, based on conversations with his staff members, openly gay and lesbian service members, and his wife and daughters.

"I have to tell you that over the course of several years as I have talked to friends and family and neighbors, when I think about members of my own staff who are in incredibly committed monogamous relationships, same-sex relationships, who are raising kids together; when I think about those soldiers or airmen or marines or sailors who are out there fighting on my behalf and yet feel constrained, even now that 'don't ask, don't tell' is gone, because they are not able to commit themselves in a marriage, 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," Obama told Roberts in an interview to appear on ABC's "Good Morning America" Thursday.

Excerpts of the interview will air tonight on ABC's "World News With Diane Sawyer" and " Nightline."

The president stressed that this is a personal position, and that he still supports the concept of states' deciding the issue on their own. But he said he's confident that more Americans will grow comfortable with gays and lesbians getting married, citing his own daughters' comfort with the concept.

Watch Obama Explains Support for Same Sex Marriage

"It's interesting, some of this is also generational," the president continued. "You know when I go to college campuses, sometimes I talk to 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 or, you know, sexual orientation, that they believe in equality. They are much more comfortable with it. You know, Malia and Sasha, they have friends whose parents are same-sex couples. There have been times where Michelle and I have been sitting around the dinner table and we're talking about their friends and their parents and Malia and Sasha, it wouldn't dawn on them that somehow their friends' parents would be treated differently. It doesn't make sense to them and, frankly, that's the kind of thing that prompts a change in perspective."

Roberts asked the president whether first lady Michelle Obama was involved in his decision. Obama said she was, and he talked specifically about his own faith.

Watch Michelle Obama's View on Same-Sex Marriage

"This is something that, you know, we've talked about over the years and she, you know, she feels the same way, she feels the same way that I do. And that is that, in the end the values that I care most deeply about and she cares most deeply about is how we treat other people and, you know, I, you know, we are both practicing Christians and obviously this position may be considered to put us at odds with the views of others but, you know, 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, you know, treat others the way you would want to be treated. And I think 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 as a dad and a husband and, hopefully, the better I'll be as president."

Watch Obama Says Kids Helped Him Evolve

Previously, Obama has moved in the direction of supporting same-sex marriage but has consistently stopped short of outright backing it.

Instead, he has voiced support for civil unions for gay and lesbian couples that provide the rights and benefits enjoyed by married couples, although not defined as "marriage." At the same time, the president has opposed efforts to ban gay marriage at the state level, saying that he did not favor attempts to strip rights away from gay and lesbian couples.

The president's position became a flashpoint this week, when Vice President Joe Biden pronounced himself "absolutely comfortable" with allowing same-sex couples to wed.

Obama aides insisted there was no daylight between the positions held by the president and his vice president when it comes to legal rights, but as other prominent Democrats also weighed in favor of gay marriage, the disconnect became difficult for the White House to explain away.

President Obama's Evolution on Same-Sex Marriage

The announcement completes a turnabout for the president, who has opposed gay marriage throughout his career in national politics. In 1996, as a state Senate candidate, he indicated support for gay marriage in a questionnaire, but Obama aides later disavowed it and said it did not reflect the candidate's position.

In 2004, as a candidate for the U.S. Senate, he cited his own religion in framing his views: "I'm a Christian. I do believe that tradition and my religious beliefs say that marriage is something sanctified between a man and a woman."

He maintained that position through his 2008 presidential campaign, and through his term as president, until today.

As president in 2010, Obama told ABC News' Jake Tapper that his feelings about gay marriage were "constantly evolving. I struggle with this." A year later, the president told ABC's George Stephanopoulos, "I'm still working on it."

"I probably won't make news right now, George," Obama said in October 2011. "But I think that there's no doubt that as I see friends, families, children of gay couples who are thriving, you know, that has an impact on how I think about these issues."

Obama's decision has political connotations for the fall. The issue divides elements of the Democratic base, with liberals and gay-rights groups eager to see the president go further, but with gay marriage far less popular among African-American voters.

Just yesterday, in North Carolina, voters overwhelmingly approved a constitutional ban on gay marriage. Obama carried North Carolina in 2008, and its status as a 2012 battleground state was guaranteed by Democrats' decision to hold their convention in Charlotte this summer.

Obama's likely Republican opponent, Mitt Romney , opposes gay marriage, and fought his state's highest court, as governor, when Massachusetts became the first state to legalize gay marriage in 2004. Romney said on the campaign trail Monday that he continues to oppose gay marriage.

"My view is that marriage is a relationship between a man and a woman," Romney said. "That's the position I've had for some time, and I don't intend to make any adjustments at this point. … Or ever, by the way."