Update (3:34 p.m. EDT): More from GMA's write-up of the interview, in which the president explained First Lady Michelle Obama's involvement in reaching his position:

“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.”

Update (3:31 p.m. EDT): Media Matters' Eric Boehlert called ABC's exclusive on this story the "revenge of network news." In a post at The Times' live blog, Stelter suggested some reasons why the White House went with ABC and with Roberts, specifically, instead of a speech.

Officials at ABC and at competing networks say they don't know for sure, but that several factors likely influenced the administration's decision. Among them: Ms. Roberts, as her fans already know, is a woman, an African American and a Christian. Polling data indicates that African Americans and regular churchgoers are more prone to oppose gay marriage than other groups of people.

Update (3:20 p.m. EDT): The decision to publicly endorse gay marriage was not a simple one. The New York Times' correspondent Jeff Zeleny tweets: "Some Obama advisers were divided on decision to support same-sex marriage, but concluded his brand has been damaged enough by hedging." Meanwhile, Ambinder points out that "gay money is bankrolling Obama's re-election as much as any other source. I think he's gonna raise a lot today." And Tau quotes a statement from Republican gay rights group GOProud: "Obama has finally come around to the Dick Cheney position on marriage equality."

Update (3:13 p.m. EDT): ABC News only aired a short snippet from the Obama interview, but there's a lot more in print over at GMA, starting with the money quote at greater length:

"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.”

Update (3:02 p.m. EDT): It's official. The president told Roberts: "I think same-sex couples should be able to get married." Mediaite has the video:

Update (2:54 p.m. EDT): Here it is, from ABC: "Obama: Same Sex Marriage Should be legal"

Update (2:53 p.m. EDT): Matthew Keys, at Reuters, caught an intriguing details in ABC's preparation for its coverage:

Update (2:32 p.m. EDT): The network will air the clips at noon, Pacific time, tweets Variety political reporter Ted Johnson. That means 3 p.m., Eastern -- superseding General Hospital.

Update (1:25 p.m. EDT): The Times' Brian Stelter tweets that excerpts from the interview, scheduled for 1:30 p.m., will air immediately after it concludes. "My TV is tuned to the ABC network," he says.

Original: In less than an hour, President Barack Obama is scheduled to sit down for an interview with ABC News' Robin Roberts, and with the White House press briefing canceled, the big news expected to come from that talk is some clarification of Obama's position on gay marriage.

Politico's Byron Tau noted on Twitter that the White House canceling a press briefing "happens on days with big news." And West Wing Report tweeted that "Rumors swirling that President will make his views on gay marriage clear - no more 'evolving.' " Before the briefing was officially canceled, The Atlantic contributing editor Marc Ambinder tweeted "So do I think President Obama will endorse same-sex marriage today? Yes. Yes I do."

The interview, which Politico's Jennifer Epstein called "hastily scheduled," comes the day after North Carolina voted to ban gay marriage and as White House spokesman Jay Carney has fielded more than 50 questions on the president's position on the issue over the last two days. The New York Times Caucus blog notes that Obama has been "under mounting pressure to clarify his thinking on same-sex marriage after top aides publicly embraced it in recent days." Since Vice President Joe Biden said he was "absolutely comfortable" with it on Sunday, Secretary of Education Arne Duncan has come out in favor. Gay rights advocates told Epstein they expected the president to come out in favor of same-sex marriage.

But in spite of all the finger-crossing and predictions, the note of caution from The New Yorker's Ryan Lizza is worth leaving off on as we wait for news from the interview, scheduled to air in full on Good Morning America Thursday but with highlights appearing on ABC's World News with Diane Sawyer Wednesday night: "A lot of confident predictions on twitter/cable about how pro-SSM position will impact election. Truth is we can't/don't really know." One thing that does feel like a safe bet: This interview should give Good Morning America a ratings edge over rival Today.