Updated 4:50 p.m. ET- President Barack Obama endorsed the right of same-sex couples to marry on Wednesday, a landmark pronouncement made in light of mounting pressure from gay rights advocates.

Obama became the first U.S. president to back the right of gay and lesbian couples to marry, a reversal from views expressed during the 2008 campaign, when he said he opposed same-sex marriage but favored civil unions as an alternative.

Related: The ‘evolution’ of Obama’s stance on gay marriage

Obama told ABC News that, after reflection, he had "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."

Related: Romney calls marriage 'tender' issue, skirts Obama remarks

In making his announcement, Obama completes what he had described as an “evolution” in his views on this issue, hastened by growing fervor this week involving gay rights. The growing pressure was capped Tuesday by North Carolina voters’ approval of a constitutional amendment banning not only same-sex marriages, but civil unions for gay and lesbian couples, as well.

Obama’s shift not only speaks to a broad swath of the electorate, which has exhibited increasing acceptance of same-sex marriage in opinion polls, but also gay and lesbian voters who compose a core part of Obama’s base, and have been major fundraisers for his re-election.

ABC News

President Barack Obama appears in an interview with ABC News on Wednesday, speaking in support of gay marriage. "It is important for me to go ahead and affirm that I think same sex couples should be able to get married," the president said.

Obama explained that he had hesitated in fully supporting same-sex marriage because he thought civil unions would be sufficient.

"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 told ABC.

The president had found himself under increasing pressure this week to state his position unequivocally after Vice President Joe Biden voiced support for same-sex marriage.

"I am absolutely comfortable with the fact that men marrying men, women marrying women, and heterosexual men and women marrying another are entitled to the same exact rights, all the civil rights, all the civil liberties," Biden said on NBC’s "Meet the Press." "And quite frankly, I don't see much of a distinction beyond that."

While the White House emphasized that Biden’s position wasn’t representative of the entire administration, Education Secretary Arne Duncan’s pronouncement Monday on MSNBC’s “Morning Joe” in support of same-sex marriage added to pressure on the president.

“I have no update on the president's personal views,” press secretary Jay Carney said repeatedly at Monday’s White House press briefing in reference to the president’s self-styled “evolution” on gay marriage.

As a result, Obama has risked fallout among his political base. The Washington Post reported this week that gay and liberal donors had threatened to withhold financial support for the president or a super PAC due to his refusal to sign an executive order barring discrimination of gays and lesbians in federal contracting.

And Obama was expected, too, to encounter frustration at a major Hollywood fundraiser this week at the home of actor George Clooney.

The overwhelming approval, too, of the measure, which Obama had opposed, in North Carolina -- a key swing state -- heightened speculation that the president might address the issue.

RELATED: North Carolina approves ban on same-sex marriage

GOP presidential candidate Mitt Romney re-iterated his opposition to both same-sex marriage and civil unions on Tuesday.

"I have the same view on marriage that I had when I was governor and that I've expressed many times," he said following the president's announcement. "I believe marriage is a relationship between a man and a woman."

Earlier, he told KDVR-TV in Denver: "I do not favor marriage between people of the same gender, and I do not favor civil unions if they are identical to marriage other than by name ... My view is the domestic partnership benefits, hospital visitation rights, and the like are appropriate but that the others are not."

Obama has faced tremendous pressure throughout his administration to advance gay rights.

Among his earliest acts as president included signing an executive order extending benefits to federal employees in same-sex partnerships in 2009. Obama also ordered the government to stop defending the Defense of Marriage Act -- the 1996 laws allowing states to refuse to recognize same-sex marriages in other states -- in court.

The administration’s crowning achievement on gay rights came more methodically, though -- sometimes to the frustration of advocates for same-sex rights.

Obama signed the repeal of “Don’t Ask, Don’t Tell” -- the military’s ban on openly gay or lesbian service members -- into law in December 2010. But the repeal came after months of legislative wrangling, and the president’s refusal to sign a simple order to make the change. And even after Obama signed the law, the implementation took months.

FIRST READ: Is Obama's gay marriage stance all about suburban voters?

Same-sex marriage is hardly the hot-button issue it was compared to the last decade, though. Support for it now eclipses opposition; 49 percent of Americans said that favor allowing gays and lesbians to marry, according to the March NBC News/Wall Street Journal poll, while 40 percent oppose it. (In October 2009, 49 percent opposed same-sex marriages, while 41 percent supported them.)

Opinion has shifted especially among independent voters, who back marriage rights 46 percent to 37 percent. About three in 10 Republicans said they, too, support same-sex marriage.

However, of the 18 states making composing the “toss-up” or “lean” categories in NBC’s battleground map, 10 have banned same-sex marriage and civil unions outright, either by constitutional amendment or statute. Just two -- Iowa and New Hampshire -- have legalized gay marriage outright, while other states operate in more nebulous space when it comes to gay and lesbian couples.