Barack Obama’s evolution is complete.

The president of the United States has just endorsed gay marriage—putting him not just in stark opposition to Mitt Romney but to Americans who firmly oppose it and have defeated every state referendum designed to legalize it.

Obama made the declaration to ABC News just three days after Joe Biden said he was “absolutely comfortable” with men marrying men and women marrying women—which prompted some administration officials to suggest the vice president was freelancing and that his remarks had not been cleared in advance.

While Obama insisted he was speaking personally, rather than mounting a campaign to change state laws: "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 ABC’s Robin Roberts. The full interview will appear on Thursday’s Good Morning America, with excerpts Wednesday evening on World News.

At the risk of resorting to hyperbole, this is a political earthquake that shakes the landscape by putting a divisive culture-war issue front and center. The betting had been that while Biden and others would signal support on a wink-wink basis, Obama would play it safe and take no position until after the election.

This was no slip of the tongue; Obama intended to make news when his staff hastily arranged the interview.

The bold move will undoubtedly garner the president support among many younger people, who have a more benign view of same-sex marriage than their elders, and gay Americans who have long felt treated like second-class citizens. A half-dozen states, most recently New York and Maryland, have adopted gay marriage.
But by taking a position that would have been politically unthinkable even four years ago, Obama is inviting the ire of roughly half the population that opposes gay marriage, many of them for religious reasons. North Carolina resoundingly defeated a referendum to legalize such marriages on Tuesday.

Romney supports a constitutional amendment to ban same-sex marriage. But with his focus on the economy, he has not seemed anxious to engage on the issue—and faces a major decision on whether to spotlight his opposition to what proponents call marriage equality.