Then there were 10. No, wait. Nine.

In the past month, the rapid rise in public support for same-sex marriage has left Supreme Court justices perplexed and some political veterans, such as Hillary Rodham Clinton, struggling to keep up.

But there may be no better measure of this historic change and its disorienting speed than what has happened among the Democrats of the U.S. Senate. They are often representatives of an urban party in rural places. That makes them the zebras of Washington’s little world: perpetually vulnerable, sniffing the wind.

As recently as the last election, 17 of those Democrats did not support same-sex marriage. By this week, however, the number was smaller.

And getting more so by the day.

“After much thought and prayer, I have come to my own personal conclusion that we shouldn’t tell people who they can love or who they can marry,” Sen. Kay Hagan (D-N.C.) said Wednesday in a statement posted on her Facebook page.

A day before, Sen. Jon Tester (Mont.) had announced his own change of heart. A day before that, it was Sen. Mark R. Warner (Va.). A day before that, Sen. Claire McCaskill (Mo.) posted her announcement on a Tumblr page, above a frittata recipe.

After Hagan, nine Democrats left.

And on Wednesday, even that group seemed to show limited enthusiasm for the cause of limiting marriage to unions of one man and one woman.

One senator’s office issued a terse statement, reiterating his support. Two senators said they wanted to leave the matter to the states. One was described as “evolving” — although it was not clear into what. The other five just didn’t call back.

It’s easy to see why they might be disoriented. In U.S. politics, the cause of same-sex marriage has still lost far more often than it has won. In all, 38 states have passed laws outlawing it. And it wasn’t until last fall — in Maryland, Washington and Maine — that voters in any state actually voted to approve it.

Not five months later, however, the world looks different. A recent Washington Post/ABC News poll showed a clear majority in favor of legalizing the marriages. It also showed that 81 percent of adults under 30 — today’s voters and tomorrow’s — support it.

The Post’s Bob Barnes explains what went on inside the Supreme Court on Wednesday as the justices appeared skeptical during the oral arguments for the Defense of Marriage Act. (JulieAnn McKellogg and Jayne Orenstein/The Washington Post)

For many Democrats, support for same-sex marriage no longer seems like an option. It seems like an obligation.

“Many more family members have come forward, whether it’s somebody’s sister, brother, cousin, uncle, mother, father, whatever it is, have come forward as to their interest in marriage equality, and that’s been then translated into our popular culture. . . . So, we’re in a different place,” said House Minority Leader Nancy Pelosi (D-Calif.) on Wednesday, describing how she’s seen other lawmakers change their opinions.

Pelosi, who has supported same-sex marriage since 1987, spoke to reporters after Wednesday’s arguments at the Supreme Court, which was examining the 1996 Defense of Marriage Act.

Republicans, of course, are going through their own self-examination on this issue. This month, Sen. Rob Portman (Ohio) announced that he had reversed his earlier position and now supported gay marriage. Portman cited his son’s decision two years earlier to come out as gay.

Today, Portman is the only Senate Republican to openly support same-sex marriage. But the GOP caucus’s tone has still shifted noticeably: As recently in 2006, more than 40 Republican senators (and two Democrats) supported a failed effort to amend the Constitution and ban same-sex marriages nationwide.

Today, that effort has been largely abandoned. In fact, two of the Republican senators seen as contenders for the 2016 presidential nomination — Marco Rubio (Fla.) and Rand Paul (Ky.) — have taken an opposite approach, saying the question should be left to individual states.

Across the Senate’s center aisle, many Democrats have shifted further. And faster.

Last fall, as Tester faced a close reelection race, his spokeswoman told Bloomberg News that Tester “supports civil unions for committed same-sex couples, but in Montana, marriage is between one man and one woman.”

This week, safely reelected, Tester saw it differently.

“I’m proud to support marriage equality because no one should be able to tell a Montanan or any American who they can love and who they can marry,” he wrote on his Facebook page.

On Wednesday, The Post sought to survey the other Democratic senators that Tester left behind. At the office of Sen. Mark Pryor (D-Ark.), a conservative Democrat facing reelection in 2014, a spokeswoman said Pryor had not changed his position. He still opposes same-sex marriage. He still supports the Defense of Marriage Act, which withholds federal recognition from gay couples who have been wed under state law.

Two other senators, Tim Johnson (S.D.) and Heidi Heitkamp (N.D.), took the same hands-off position as their Republican counterparts, Rubio and Paul.

“He believes that same-sex marriage should be left up to the individual states,” a spokesman for Johnson said. Was there any pressure to change that stance? “I’m not aware of any pressure,” the spokesman said.

Another Democrat, Sen. Thomas R. Carper (Del.) seemed to be in the middle of his own struggle on the issue.

Carper has not explicitly supported same-sex marriages. But he has supported Delaware’s policy of offering civil unions to gay couples. And he signed off on a legal brief that urged the Supreme Court to invalidate the Defense of Marriage Act.

“He has also said that he would vote to repeal [the act],” a spokeswoman for Carper said. “Like many Americans including Presidents Obama and Clinton, Senator Carper’s views on this issue have evolved, and continue to evolve. He continues to give this issue a great deal of consideration.”

According to the activist group Human Rights Campaign, 46 of 55 Senate Democrats and independents now support same-sex marriage, compared to last fall, when supporters numbered 36 out of 53.

Spokesmen for Sens. Robert P. Casey Jr. (D-Pa.), Bill Nelson (D-Fla.), Joe Manchin III (D-W.Va.), Joe Donnelly (D-Ind.) and Mary Landrieu (D-La.) did not respond to requests for comment.

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