It started Sunday night with Claire McCaskill (D-Mo.). “I have come to the conclusion that our government should not limit the right to marry based on who you love,” she wrote on her Tumblr. Or maybe it was last Thursday, when Sen. Tim Kaine (D-Va.) casually told a reporter he'd (privately) backed gay marriage since 2006. Mark Warner (D-Va.) and Mark Begich (D-Alaska) chimed in Monday, just as Sens. Tim Johnson (D-S.D.) and Jay Rockefeller (D-W.V.) announced that they no longer support the Defense of Marriage Act. Sen. Jon Tester (D-Mont.) joined them Tuesday with a Facebook post, followed by Sen. Kay Hagan (D-N.C.) on Wednesday.

All over the country, Democratic senators who have avoided the gay marriage question for years are suddenly coming out.

With the exception of Warner, none of these senators are considered potential presidential contenders in 2016. So unlike Hillary Clinton, they don't have to worry about a national Democratic primary in which support for gay marriage will likely be a litmus test. What gives?

Like Republicans, they can read the polls. In the most recent Washington Post-ABC News poll, six in 10 Americans support the legalization of gay marriage, including 80 percent of those ages 18 to 29 supportive of gay marriage. Eight years ago, in 2004, just 41 percent of Americans in the Post-ABC survey backed gay marriage. That was the year 11 states banned gay unions by ballot measure -- and spooked Democrats in the process. With the Supreme Court hearing arguments on the constitutionality of gay marriage today and Wednesday, Democrats have decided that this is the moment for the history books. And they want to go down leading, not lagging.

"I think many of the Democratic senators coming out in favor of marriage equality have long records of fighting for equal rights and now see an opening to embrace a position that strengthens their commitment," a Democratic strategist said.

Johnson and Rockefeller are retiring senators in red states. They aren't concerned with reelection and are can think solely about their legacies. McCaskill and Tester are not up for reelection for another five years; they can bank with relative safety on gay marriage's decreasing power as a wedge issue. Begich is in a trickier spot; he faces voters next year. But polling suggests that support for same-sex unions has grown faster in Alaska than in Missouri.

Warner, like Clinton, might run for president and needs to be in line with the Democratic base. While he's also up for reelection in 2014, the political risk is not that high -- a recent Post poll found that 49 percent of Virginians back gay marriage, including 54 percent of independents. Only 40 percent are opposed.

"Elected officials are always assessing what the condition on the ground in their states are, where the country is, and what the right thing to do is," said Marc Solomon, national campaign director for the gay marriage advocacy group Freedom to Marry. "I think it's a really strong affirmation of where we are in this movement."

Likewise, Human Rights Campaign spokesman Fred Sainz called the senators' statements "perfect proof" that the Prop 8 case was sparking conversations across the country, and that "thoughtful individuals can evolve" on gay marriage.

There's also a financial factor. Gay donors played a major role in President Obama's reelection. The pressure on Obama to embrace marriage showed that activists in the gay community were no longer satisfied with anything less. Donations flooded in after his change of heart; donors threatened to withhold funds over other gay rights issues. Vulnerable senators looking for campaign cash are surely keeping all that in mind.

Sen. Rob Portman (R-Ohio) might also play a role here. While his announcement of support for same-sex marriage has not led any other Republican lawmakers to step out, having a sliver of bipartisan consensus on the issue gives cover to moderate Democrats.

But more than anything, the rush to back gay marriage shows how successful same-sex marriage activists have been at framing this issue as one of basic rights. Even after Newtown, conservative Democrats have taken little heat for opposing gun control measures -- it wasn't a turning point many thought it would be. But for gay marriage, that point is now.