Some of the holdouts are starting to feel heat from frustrated voters at home. | AP Photos Holdout Dems on gay marriage

There’s a new lonely hearts club in Washington: Senate Democrats who haven’t endorsed gay marriage.

They’ve been left behind, stranded by a new wave of Democrats who, for a combination of political and personal reasons, have jumped on the same-sex matrimony bandwagon. Sen. Kay Hagan, a North Carolina Democrat and top Republican target in the 2014 midterm election, went public with her support for gay marriage on Wednesday, joining Jon Tester of Montana, Jay Rockefeller of West Virginia, Claire McCaskill of Missouri and Mark Warner of Virginia as recent converts to the side of same-sex marriage.


Not so long ago, a Democrat was considered progressive if he or she voted against a constitutional amendment banning gay marriage. And during the 2008 presidential campaign, it was fashionable for Democrats to say they backed domestic-partner benefits and civil unions, but not gay marriage.

That was the safe, risk-averse place to be. Not anymore.

Only nine of the 55 Democratic senators don’t currently back gay marriage — and some of them are starting to feel the heat from frustrated base voters at home and from the example of their peers in Washington. President Barack Obama, the Democratic leaders in the House and Senate, and all of the serious potential contenders for the Democratic presidential nomination in 2016, are now firmly in support of gay marriage. That makes it increasingly difficult for Democratic lawmakers to defend positions that stop short of endorsing what proponents call “marriage equality.”

( PHOTOS: Democratic senators who have not endorsed gay marriage)

They’re seen as being on the wrong side of history — even bigoted — in the eyes of progressives, establishment Democrats and no small number of Republicans and political independents.

“If he feels we are not equal, then that is something he has to live with,” Norman Spector, president of the Barbara Gittings Delaware Stonewall Democrats, said of homestate Sen. Tom Carper’s refusal to endorse same-sex marriage. But Spector said he continues to back Carper, who just won reelection in 2012, noting that there’s just one major issue on which they disagree.

None of the Democratic holdouts are likely to lose a party primary over failure to back same-sex marriage in 2014, but the marriage-is-between-a-man-and-a-woman crowd risks alienating the big-time donors and grass-roots activists who make gay rights a priority.

( AUDIO: Supreme Court oral arguments on DOMA)

“Marriage equality is a complex issue with strong feelings on both sides, and I have a great deal of respect for varying opinions on the issue,” Hagan said Wednesday. “After much thought and prayer, I have come to my own conclusion that we shouldn’t tell people who they can love or who they can marry.”

It might seem like a bad move for Hagan because North Carolina voters just adopted a same-sex marriage ban by a 61 percent to 39 percent margin last year. And it might still prove to be poor home state politics. But Hagan had little to gain by staying on the sidelines on same-sex marriage at the federal level after having opposed the state ballot initiative last year. She won’t win over the conservatives who thought she was wrong then, and by embracing gay marriage now she may able to attract contributions, as well as campaign volunteers who might otherwise have viewed her with suspicion.

( Also on POLITICO: Pelosi: House GOP 'irresponsible' on DOMA)

The calculation varies from state to state and from lawmaker to lawmaker — and each seems to weigh a slightly different balance of the concerns of their constituents, their own personal feelings and raw political instincts.

The rush to follow the pack has been intensified in the past couple of days. The Huffington Post created a list of Senate Democrats who don’t back gay marriage on Tuesday but had trouble keeping it up to date, as Tester announced his support later that day, and Hagan joined on Wednesday.

National Republicans are more inclined to tweak Democrats for flip-flopping than attack them for supporting gay marriage.

“Voters in each of these states are going to have to decide what they think about the wavering positions of their officials,” National Republican Senatorial Committee spokesman Brad Dayspring said.

The nine Democrats who haven’t endorsed gay marriage are Tim Johnson of South Dakota, Heidi Heitkamp of North Dakota, Mary Landrieu of Lousiana, Bill Nelson of Florida, Joe Manchin of West Virginia, Carper, Mark Pryor of Arkansas, Joe Donnelly of Indiana and Bob Casey of Pennsylvania.

Those Democrats now find themselves to the right of former Vice President Dick Cheney, former Republican National Committee Chairman Ken Mehlman and Republican Sen. Rob Portman of Ohio on the major social issue of the moment.

The new middle ground most of them are seeking leaves room for the kind of “evolution” President Barack Obama said he underwent in moving from an endorsement of civil unions to one of gay marriage.

Donnelly, elected in 2012 in traditionally Republican Indiana, takes the position that marriage is between a man and a woman. But he’s leaving himself some wiggle room. Spokesman Ben Ray told WRTV in Indianapolis that Donnelly will “continue to review the issue” while the Supreme Court is considering a pair of same-sex marriage cases.

Using similar language, Landrieu hasn’t ruled out endorsing same-sex marriage at some point.

“We’ll have to see what the Supreme Court says about gay marriage,” Landrieu told POLITICO. “And I just think that people’s views about it are changing quite rapidly, a more progressive position. I’m just going to continue to talk to the people of my state.”

The third-term Louisianan is a perfect example of a senator who went from being perceived as a bold proponent of gay rights to being a bit behind the progressive curve without changing her position. The last time she ran for reelection, in 2008, she was the leading recipient of campaign cash from the Human Rights Campaign, according to the Center for Responsive Politics, and she has won praise in the past from gay-rights activists for voting against a constitutional amendment banning same-sex marriage.

In February, Public Policy Polling found that 59 percent of respondents in Louisiana believed gay marriage should be illegal, while only 29 percent said it should be legal.

Longtime Landrieu backer Jody Gates, a member of the political committee of the New Orleans-based Forum for Equality, said she plans to keep prodding her senator on same-sex marriage but understands the political realities that Landrieu faces.

“If she came out in support of gay marriage, I don’t know if she could get elected in this state,” Gates said. “Any of the Republicans would be aggressively anti-anything that might … benefit the LGBT community.”

Add Carper to the list of politicians who might “evolve” to support for same-sex marriage.

“Like many Americans, including Presidents Obama and Clinton, Sen. Carper’s views on this issue have evolved, and continue to evolve,” Carper spokeswoman Emily Spain said in a statement. “He continues to give this issue a great deal of consideration.”

Heitkamp has said she believes it is a matter for each state to decide.

But not all of the Democrats are leaving themselves space: Manchin, Pryor, Nelson and Johnson have all indicated they remain dead-set against same-sex marriage.

— Ginger Gibson contributed to this story