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In 2011, only 15 senators backed same-sex marriage. Now 49 do. UPDATE: Now 51!

Today, Mark Kirk and Tom Carper became the latest senators to endorse same-sex marriage, as members of the chamber seem to be falling over themselves to do in recent weeks. Just in the past month, nine members, including Kirk and Carper, have reversed course on the issue. The changes followed both Rob Portman's announcement that he supports same-sex marriage, and the Supreme Court arguments on Proposition 8 and the Defense of Marriage Act.

By my count, that makes 49 members of the Senate, plus Vice President Biden, the Senate president, who support same-sex marriage. How big a change is that? The first senator to hold this position is Ron Wyden, who has supported same-sex marriage since taking office in 1996. But for many years after that, change was slow. Here are the numbers, based on my counts (see this spreadsheet for my numbers):

It's basically an exponential increase.

These numbers are a bit rough around the edges. I counted Carol Moseley-Braun and Ted Kennedy's speeches opposing DOMA as expressions of support for same-sex marriage, which I think is clear from the text; your interpretation may differ, and they never say "I support same-sex marriage" in as many words. Former senator Mark Dayton expressed support for civil unions for all couples and the abolition of marriage altogether in 2006, which I didn't count. I counted sponsorship of the Respect for Marriage Act, which repeals DOMA, as an expression of support for same-sex marriage. I think that's a fair inference, especially given that many co-sponsors have gotten more explicit about their support for same-sex marriage since (see this comment from Tom Udall, for instance, a year after cosponsoring the Respect for Marriage Act). Finally, I am still waiting to hear back from Rhode Island governor Lincoln Chafee's office as to when during his tenure as a Republican senator he endorsed same-sex marriage, so I left him off for now.

But however you cut it, support for same-sex marriage is spreading through the Senate at a very rapid pace.

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