A majority of U.S. senators now say they back the right of gays and lesbians to marry, including three Democrats who announced their support within 24 hours.
Senators Joe Donnelly of Indiana, Heidi Heitkamp of North Dakota and Bill Nelson of Florida issued statements reversing their earlier opposition and endorsing same-sex marriage. That puts 53 senators on record in favor of extending marriage to couples of the same sex, including Republicans Mark Kirk of Illinois and Rob Portman of Ohio.
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"This tidal wave of support for marriage equality in Congress reflects what we're seeing happening in the lives of everyday Americans around the country," said Michael Cole-Schwartz, a spokesman for the Human Rights Campaign, an advocacy group for gay and lesbian rights. "People understand there's no reason to treat gay and lesbian couples differently than anyone else and it's crystal clear that the country is ready."
Four members of the Senate Democratic caucus -- Mary Landrieu of Louisiana, Tim Johnson of South Dakota, Joe Manchin of West Virginia and Mark Pryor of Arkansas -- remain in the opposition camp. Landrieu and Pryor face re-election in 2014. Johnson is retiring at the end of his current term in 2015.
Advocates said momentum has been building since President Barack Obama and the Democratic Party platform backed same-sex marriage, as did voters in Maine, Maryland and Washington in November. A March poll by Hamden, Connecticut-based Quinnipiac University showed U.S. voters backing same-sex marriage, 47 percent to 43 percent, a reversal from its July 2008 survey in which 55 percent were opposed and 36 percent were in favor.
The U.S. Supreme Court last month heard challenges to a California referendum that outlawed same-sex marriage and the federal Defense of Marriage Act, which prevents the federal government from providing benefits to married couples of the same sex. A ruling is likely by June.
"The country is on an evolution on this issue as are all of our elected leaders," Cole-Schwartz said. "Over time, as people continue to think about this, and as public opinion continues to go in our favor, you're going to see members change."
Heitkamp's statement said she concluded that the federal government "should no longer discriminate against people who want to make lifelong, loving commitments to each other or interfere in personal, private and intimate relationships." She said she considered "the ability of anyone to marry as a logical extension of this belief."
Nelson's statement, sent to the Tampa Bay Times, said: "The civil rights and responsibilities for one must pertain to all. Thus, to discriminate against one class and not another is wrong for me. If we are endowed by our creator with rights, then why shouldn't those be attainable by gays and lesbians?"
Donnelly said he had been rethinking his previous votes and positions.
"In doing so, I have concluded that the right thing to do is to support marriage equality for all," his statement said. "We are a stronger country when we draw on the strengths of all Americans."
Two of the three senators, Heitkamp and Donnelly, represent states that Obama lost last fall.
"Red-state Democrats now see that this is not just a morally right place to be but a politically smart place to be," said Marc Solomon, national campaign director for New York-based Freedom to Marry.
Supporters of same-sex marriage are calling and writing to the legislative offices of holdouts. The Washington-based Human Rights Campaign has modified its congressional scorecard to include a lawmaker's position same-sex marriage in addition to his or her votes on other issues of concern to gays and lesbians.
Solomon said that gay and lesbian couples are trying to tell their stories directly to lawmakers and their aides, and through media coverage of unsuccessful efforts to get marriage licenses at county clerks' offices.
"Our approach is to continue to make our case through the telling of stories of loving and committed couples," he said.
Johnson's spokesman, Perry Plumart; Amber McDowell, a spokeswoman for Landrieu; and Michael Teague, a spokesman for Pryor, didn't respond to phone calls and e-mailed requests for comment.
Manchin said he would continue to oppose same-sex marriage.
"I believe that a marriage is a union between one man and one woman," he said in a statement. "My beliefs are guided by my faith, and I support the Defense of Marriage Act."