At the same time, Obama aides sought to gain the upper hand on the issue with independent voters by highlighting Romney's record on gay rights. Aides argued there was a clear distinction between the Republican candidate and Obama, who repealed the military's ban on openly gay service members and ordered his administration to stop enforcing the Defense of Marriage Act, which defines marriage as between a man and a woman.
Romney favors a constitutional amendment to ban gay marriage, saying the policy should be set federally, not by states. Some conservatives have questioned Romney's commitment to that position, noting that during his 1994 Senate run, he said he supported "full equality" for gays and lesbians.
To put such doubts to rest, Romney told an Ohio television station Monday that he believes "marriage is between a man and a woman, and that's a position I've had for some time and I don't intend to make any adjustments at this point — or ever, by the way."
Many gay rights advocates and people close to Obama's campaign suggest it is no longer a matter of if, but when the president voices his support for same-sex marriage. The hope among some Democrats, and the fear among some Republicans, is that Obama is simply waiting until after the election to state his views publicly.
Obama has acknowledged he will have more freedom in some areas following the November contest. He was overheard telling outgoing Russian President Dmitry Medvedev in March that "after my election I have more flexibility." In that instance, Obama was speaking about the issue of missile defense.
Richard Socarides, a gay rights supporter who was a White House official in the Clinton administration, said Duncan's and Biden's comments this week may prevent Obama from being able to stay quiet on gay marriage before Election Day.
"It becomes increasingly difficult to finesse this for very much longer," Socarides said. "The president is going to get asked about this every time there's an opportunity."
One upcoming opportunity is a campaign fundraiser Obama is scheduled to headline with gay and lesbian supporters in early June.
The issue also is expected to surface at the Democratic convention in September, where many party leaders want to include support for gay marriage in the official platform. Caroline Kennedy, one of Obama's campaign co-chairs, voiced her support for that effort on Monday.
Gay marriage is legal in six states, plus the District of Columbia.
Obama's reluctance to embrace gay marriage has increasingly put him at odds with a majority of Americans. A Washington Post/ABC News poll from March found that 52 percent felt it should be legal for gay and lesbian couples to get married, while 43 percent said it should be illegal.
Support for gay marriage is highest among Democrats, with 64 percent supportive of the issue. Just over half of independents — 54 percent — back legalized gay marriage, according to the Post/ABC poll. Support among Republicans is the lowest, at 39 percent.
Associated Press writer Philip Elliott contributed to this report.
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