So who will be No. 54? How long after that announcement will the roster of senators supporting gay marriage become filibuster-proof?

Predicting an answer to the first question requires looking more closely at the 43 potential Republican senators than at the list of just four Democrats who haven’t yet endorsed the concept of same-sex marriage. So does forecasting an answer to the second question, but on that one it’s safe to say it won’t happen before the next election.

Senate support for marriage equality surged into majority territory this week when Sen. Mark S. Kirk of Illinois became the second Republican backer of same-sex marriage; he was joined by five centrist Democrats, all of whom just won an election and are betting they’ll be squarely on the right side of history by the time they next face the voters in six years. The Democrats are third-term winners Bill Nelson of swing state Florida and Thomas R. Carper of Delaware; second-term survivor Bob Casey of bellwether Pennsylvania; and a pair of freshmen who scored upsets in GOP states, North Dakota’s Heidi Heitkamp and Indiana’s Joe Donnelly.

But the notion that a political tipping point is at hand — and that a wave of changed minds and modernized hearts will crest at the Capitol before the Supreme Court reveals itself on the gay marriage question — is swiftly put to rest by a look at the senators who remain either uncommitted or publicly on the other side.

Two are Democrats seeking re-election in 2014 in Southern states where cultural conservatism remains strong and where Mitt Romney’s margin of victory last fall was deep into double digits: Sens. Mary L. Landrieu of Louisiana and Mark Pryor of Arkansas.

Landrieu has signaled her ambivalence about denying marriage rights to gays and lesbians; Pryor has spoken unambiguously about how marriage should remain an exclusively heterosexual institution. But beyond that slight daylight, the basic dynamic of their coming campaigns will be the same: Their allegiance to the values of their states will be constantly called into question and the best way to prove their loyalties will be to buck President Barack Obama and the liberal congressional leadership whenever possible.

“My views have evolved on this. But my state has a very strong const. amendment against gay marriage & I think I have to honor that,” Landrieu told CNN’s Jim Acosta, who tweeted her stance on Friday.

She and Pryor clearly chose a while ago to make this issue one of the self-imposed wedges between them and the national party. Else they would have already endorsed same-sex marriage, as has been done in recent weeks by the other three red state Democrats running in 2014: Max Baucus of Montana, Mark Begich of Alaska and Kay Hagan of North Carolina.

A third Democrat is Sen. Tim Johnson, whose retirement would seem to free him up to join his party’s marriage equality mainstream. But South Dakota is another conservative state and the senator doesn’t want to do anything to complicate life for his son and would-be successor, Brendan, the state’s top federal prosecutor.

The fourth is Sen. Joe Manchin III, who’s won five statewide elections in the past dozen years in ever-more Republican West Virginia by carefully calibrating the line between economic populism and social conservatism. He’s not up again until 2018, but his readiness to compromise his past opposition to any more federal gun control is probably the biggest spending of political capital you’ll see from him this year. Backing gay marriage in the same season would be an ideological bridge too far.

In the end, it’s the roster of Republicans that’s going to yield the next Senate declarations of support for marriage equality — but no more than three of them, at most, before the next elections.

The likeliest switchers, by far, are the two more iconoclastic and moderate women in a caucus where all those characteristics are in short supply.

Sen. Lisa Murkowski has gone against the grain much more often since her 2010 write-in win in Alaska, after she was denied the GOP nomination. She’s already told reporters back home that her views on marriage are “evolving” — the code word used so often by politicians about to change their mind on the issue. And Begich’s switch at the start of his campaign shows that one statewide official has already concluded that Alaska’s young and libertarian conservatives can be won over to the new view.

Sen. Susan Collins’ voting record is more pro-gay rights than anyone else in the current GOP caucus. But she’s stayed on the fence on the marriage issue out of concern she could be challenged on her right in the Maine primary next year. That looks less likely to happen by the day, however, and recently she’s hinted she might have something to say after the Supreme Court’s two decisions this summer.

Collins is one of seven GOP senators who represent states Obama carried in both in 2008 and 2012, but who haven’t come out in favor of same-sex marriage. Yes, she’s the only one of them running next year, but Maine is one of nine states where gay marriage is now legal. If she doesn’t join the new Senate majority, it’s impossible to see it growing at all.