After South Dakota Sen. Tim Johnson's embrace of gay marriage last week, activists who have made the issue a litmus test for Democratic Party officeholders are cranking up the heat on the three remaining holdouts among Democrats in the Senate.
More than a dozen Democrats in the Senate have announced support for the rights of gay and lesbian couples to marry since last month, when former Secretary of State Hillary Rodham Clinton, the consensus front-runner for the party's 2016 presidential nomination, told followers she had evolved on the issue and now backs same-sex marriage.
Among Democrats, only Sens. Joe Manchin III of West Virginia, Mark L. Pryor of Arkansas and Mary L. Landrieu of Louisiana aren't on board with gay marriage.
"What was not long ago a losing position politically is becoming a winning one," said Mark Rozell, a political-science professor at George Mason University. "Look for more Democrats seeking office to play up this issue, which indeed appears to be becoming a litmus in many parts of the country."
Democratic strategist Mo Elleithee took it a step further, saying it will be "very hard for any Democrat to run for president any more if they don't openly support same-sex marriage."
"It's become the defining civil rights issue of this generation," Mr. Elleithee said. "You don't need to be the first one on board but you sure as heck don't want to be the last one, or even worse, on the outside looking in."
Ralph Reed, head of the conservative Faith and Freedom Coalition and traditional marriage supporter, said the day is coming where opponents of same-sex marriage are no longer welcome in the Democratic Party.
"Just as 20 years ago Gov. Bob Casey of Pennsylvania was excluded from speaking at Democratic National Convention because he was pro-life, the day is coming when a Democratic elected official who believes in traditional marriage will be excluded from the podium at a Democratic convention," Mr. Reed said.
The mad dash toward gay marriage started in the days leading up to the oral arguments at the Supreme Court on the constitutional challenges to California's Proposition 8, the voter referendum that banned same-sex marriage in the state, and the Defense of Marriage Act, or DOMA, which defined marriage for the purpose of federal law as a legal union between one man and one woman.
But Mr. Manchin, whose state passed a law in 2000 banning same-sex marriage, hasn't shown any signs he plans to budge. Nor have Mr. Pryor and Mrs. Landrieu, both of whom face re-election in 2014 and represent voters from Southern states that passed state constitutional amendments defining marriage as being between one man and one woman.
"I'm a lot, like other people said, my views have evolved on this," Mrs. Landrieu said this month. "But my state has a very strong constitutional amendment against gay marriage and I think I have to honor that."
Mrs. Landrieu and Mr. Pryor, though, did join the Democrat-led effort to strike down the "don't ask, don't tell" ban on gay men and lesbians serving openly in the military in 2010 a policy change that few Republicans now say should be reinstated now that it has been taken off the books. Mr. Manchin did not vote.
In 2011, a Yahoo News breakdown shows that 15 members of the Senate Democratic Caucus endorsed same-sex marriage and 20 more jumped on board in 2012 as did President Obama and Vice President Joseph R. Biden, another possible 2016 contender.
The political shift is a stark change from 1996 when both parties supported DOMA, which President Bill Clinton signed into law in 1996, explaining "I have long opposed governmental recognition of same-gender marriages."
Senate Majority Leader Harry Reid, Nevada Democrat, and his top four lieutenants in the Senate Patrick J. Leahy of Vermont, Richard J. Durbin of Illinois, Charles E. Schumer of New York and Patty Murray of Washington supported DOMA and now support same-sex marriage. Mr. Clinton now says that DOMA is unconstitutional and should be struck down.
The switch coincides with dramatic shift in public polling.
The Pew Research Center found in a March poll that 49 percent of the respondents favored same-sex marriage, while 44 percent opposed it. The same poll from 2001 showed that Americans opposed same-sex marriage by a 57 percent to 35 percent margin.
The poll showed more than a quarter of the respondents said they have changed their mind on the issue for a variety of reasons namely that they have become more comfortable with the issue thanks to friends or family members who are gay or lesbian.
"I've never seen such an overwhelming turnaround on an issue in such a short period of time," Mr. Elleithee said. "Less than 10 years ago, Republicans were trying to put Democrats on the defensive on this issue. Today, it's a complete turnaround with many conservatives squirming as Democrats stand up proudly in support."
Two Senate Republicans Rob Portman of Ohio and Mark Kirk of Illinois have announced that they support gay marriage.
But for the most part, the national Republican Party is standing by the traditional definition of marriage: one man, one woman. The 168-member Republican National Committee overwhelmingly reaffirmed that Friday, voting unanimously in favor of a resolution restating the party's traditional-marriage position.
The vote, at the RNC's spring meeting in Los Angeles, was seen by some as a rebuff of GOP Chairman Reince Priebus, who has urged the party to show greater tolerance on social issues.
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