WASHINGTON -- In a nod to a dramatic shift in public opinion, Barack Obama on Wednesday became the first sitting president to announce his support for same-sex marriage.

In a sit-down interview with ABC's Robin Roberts, Obama completed what has been a markedly long and oft-mocked evolution on the matter.

"I've always been adamant that gay and lesbian Americans should be treated fairly and equally," Obama told Roberts, in an interview that will air in full on ABC's "Good Morning America" Thursday.

(Watch ABC's entire clip below)

"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," he said.

The statement constitutes an act of political bravery on the president's behalf, as well as a major victory for the gay rights community, which has been pushing him to declare his support for marriage equality for several years. With the issue back in the news this week, the pressure intensified.

On Sunday, Vice President Joseph Biden told NBC's "Meet The Press" that he was personally comfortable with same-sex marriage, which was followed the next day by Education Secretary Arne Duncan saying the same.

The White House insisted that there was no daylight between the vice president's position and the president's, noting that Biden clarified his statement as being in reference to civil rights for gay couples. But the explanation was largely dismissed by both supporters and critics as a convenient way for the president to signal support for marriage equality without having to declare it himself.

On Tuesday evening, the state of North Carolina passed an amendment that defined marriage as a union between a man and a woman. The president expressed his disappointment with the measure, but it remained difficult to square his opposition to a measure outlawing same-sex marriage with his opposition to same-sex marriage itself.

As the political pressure continued to mount, the president finally chose to speak out Wednesday, with the White House hastily scheduling a sit-down interview.

“It’s interesting, some of this is also generational,” the president said. “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, 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.”

The president's support of same-sex marriage will have little political impact, from a practical standpoint, as much of the activity on the issue is currently occurring in the states and the courts. Already the Obama administration's Department of Justice has stopped defending the constitutionality of the Defense of Marriage Act (DOMA), which defines marriage as a legal union between a man and a woman. Legislation to overturn DOMA outright would likely be blocked by congressional Republicans.

The more promising path for same-sex marriage advocates remains a friendly hearing by the United States Supreme Court.

Still, the symbolism of Obama's remarks is hard to ignore. In becoming the first president to publicly support marriage equality, he sets the bar for its political acceptance. He also has the ability to shape public opinion further on the matter.

Of course, there may be drawbacks to such a strong expression of support. While recent polls show that popular support for marriage equality is gaining widespread acceptance, some pivotal swing states remain largely opposed to the concept. And one of them, North Carolina, remains a major target for the president's reelection campaign.

"The question is, is there a risk?" a prominent Democratic Party official who requested anonymity told The Huffington Post after Biden's remarks. "It is not nationwide [polling] we are talking about. We are talking about Virginia, North Carolina and other swing states. And we are talking about, would Karl Rove and his team stoop to using horribly grotesque and hateful tactics ... and would that peel off 10,000 votes?"

Until Wednesday, that question was hypothetical. Now, it's a critical component of the 2012 election.

Below, reactions to Obama's announcement from around the political world:

PHOTO GALLERY Politicians React To Obama On Same-Sex Marriage

Watch from ABC News: