In the U.S. Senate, it's down to the Final Four versus the Dynamic Duo.
Only four Democratic senators remain who do not support same-sex marriage. Across the aisle, there are now two Republicans who have announced their support.
The new alignments mean that a majority — 53 senators — now support a concept that 85 senators voted to ban in 1996 with the Defense of Marriage Act.
Indiana's Joe Donnelly and North Dakota's Heidi Heitkamp on Friday became the latest Democrats to join what's now the dominant position in their party. Heitkamp put out a press release, while Donnelly announced his view via a posting on Facebook, as did Montana's Jon Tester, Virginia's Mark Warner and North Carolina's Kay Hagen earlier.
The use of Facebook isn't surprising. Gay marriage is now supported by a majority of all Americans, but by an overwhelming majority of younger Americans — which is to say, those more likely to tune into social media sources.
Democrats started announcing their support of gay marriage en masse last week, when the U.S. Supreme Court heard oral arguments on two cases on the subject. But the trend was really begun by Ohio Republican Rob Portman, who declared he had changed his mind after his son announced he was gay. So far, Illinois Mark Kirk has been the only other Republican to join Portman.
As to the remaining four Democrats, it's unclear if any of them are contemplating a change of heart. South Dakota's Tim Johnson is retiring, though his son is a potential candidate to replace him in 2014. West Virginia's Joe Manchin represents a culturally conservative state, but does not face re-election until 2016.
The last two, Louisiana's Mary Landrieu and Arkansas's Mark Pryor, also represent deep red states, and are likely to face tough re-election battles next year.
S.V. Dáte is the congressional editor on NPR's Washington Desk.