Recent polling shows the country narrowly divided on the marriage question, and the president himself has said he grapples with the issue.
But initial reaction from some prominent Republicans seemed to suggest the debate is not as salient as it once was.
House Speaker John Boehner said this week only that it's not "the appropriate time" to stir up a controversial issue, and didn't even mention the word marriage.
Among those eyeing a bid for the White House in 2012, former Massachusetts Gov. Mitt Romney called the move an "unfortunate mistake."
Former Minnesota Gov. Tim Pawlenty and Huckabee both said they were "disappointed." And, former Alaska Gov. Sarah Palin and Indiana Gov. Mitch Daniels haven't said anything publicly.
Former Pennsylvania Sen. Rick Santorum, also believed to be considering a bid for president in 2012, called on Republican leaders to mount a more forceful defense of traditional marriage, in an interview with USA Today.
Obama's decision is "pretty radical, in my opinion," Santorum said, adding that Boehner should "take up the cause and argue the cases" in court.
But he said he's not surprised that some of his fellow Republicans haven't been more aggressive for that cause.
The media cast advocacy against gay marriage "in terms of bigotry, in terms of discrimination, in terms of (being) homophobic," he said.
"As a result of that, people stay away from it. They don't want to be cast in that light by the media. And besides, we all have friends who are gay. I have friends who are gay. But they respect the fact that they disagree with them on policy."