UpdateMitt Romney reaffirmed his opposition to gay marriage shortly after Obama's remarks. "I have the same view on marriage that I had when I was governor,” he said. “I believe marriage is a relationship between a man and a woman. This is a very tender and sensitive topic, as are many social issues, but I have the same view that I’ve had since running for office.” According to Politico's Ginger Gibson, when he was asked about President Obama's position, Romney said “I believe that based upon the interview that he gave today, he had changed his view, but you’re a better judge of that than I."

Original post: President Obama's endorsement of gay marriage is getting the full historical-moment treatment, sending politicians, advocacy groups and pundits racing to spin the announcement. "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 ABC News' Robin Roberts

Among gay rights supporters, the President received an outpouring of support. "A great day in our fight for civil rights. President Obama adds his support for marriage #equality #BeatuifulDay," tweeted House Minority Leader Nancy Pelosi. "President @BarackObama's announcement is a major turning point in the history of American civil rights," tweeted New York City Mayor Michael Bloomberg. The Human Rights campaign issued a statement thanking the president for supporting gay marriage, saying "Without a doubt, President Obama's statement will inspire thousands more conversations around kitchen tables and in church pews. The statement added that "We are confident that our nation will continue to move inexorably toward equality and we thank the President for leading us in that direction."

In the media, Twitter was throttled with reactions as ABC uploaded the clip to the Internet. "Well, good for Barack Obama. This is what leadership is all about," tweeted The Atlantic's James Fallows. In terms of eye-catching homepages, the left-leaning Huffington Post rolled out a Drudge-style splash page reading "I Do"  while the hard-right Fox Nation homepage attacked with the headline "OBAMA FLOP FLOPS, DECLARES WAR ON MARRIAGE." (Update: Fox Nation has since taken down the war declaration aspect of the headline). The Drudge Report, meanwhile, flashed a "Invokes daughters--again..." headline, hitting the president for bringing his daughters into a political decision. Playing up the gay innuendo, the New York Post goes big with "Obama Comes Out."

The Washington Post's Chris Cillizza says the decision was inevitable. "The fact that it was something close to an accepted fact in political circles that Obama, on a personal level, favored allowing gays to marry, it was clear that the President had to say something — and sooner rather than later," he wrote. "The movement in national polling on the issue is something rarely seen; support for gay marriage is rapidly rising even as support rapidly erodes." For a bigger breakdown of the demographics of gay marriage, National Journal's Stephanie Czekalinski has a good analysis here. One of the more unexpected reactions was Fox News' Shep Smith, who rather pointedly said the president had now entered the 21st Century.

On his blog, The Daily Beast's Andrew Sullivan has been post his favorite reader comments. "Obama isn't just some guy," the reader writes. "That the President of the United States believes in marriage equality is a big deal because it's a position he now believes is politically tenable. Of course his 'evolving' attitude about SSM was always a little contrived, but that's just how politics is." Conservative blogger Jennifer Rubin, on the other hand, doesn't think the announcement will make much  of a difference. "I think this is unlikely to change a single vote," she writes. "The vast number of Americans opposed to gay marriage are either committed conservatives who will never vote for him or African American Democrats who will vote for him no matter what. The only implication may be that Christian conservatives’ enthusiasm for Romney increases and his base-turnout problems become a non-concern."

Gay Republican groups had a rather sour tone. According to Politico's Bryan Tau, the group GOProud issue a statement saying "Obama has finally come around to the Dick Cheney position on marriage equality." Meanwhile, the Log Cabin Republicans took an even more critical stance. "LGBT Americans are right to be angry that this calculated announcement comes too late to be of any use to the people of North Carolina, or any of the other states that have addressed this issue on his watch," read the groups statement. "This administration has manipulated LGBT families for political gain as much as anybody, and after his campaign’s ridiculous contortions to deny support for marriage equality this week he does not deserve praise for an announcement that comes a day late and a dollar short.”

And then there were the memes. The Tumblr "When Obama Endorsed Same-Sex Marriage" took almost no time to pop up with a series of GIFs rejoicing in celebration. Here's an Arrested Development take:

And a Michelle Obama take for the road: