"Montanans believe in the right to make a good life for their families. How they define a family should be their business and their business alone. 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."
There are 55 members of the Senate Democratic caucus, and according to Time magazine's count, 45 of them have now publicly announced their support for marriage equality. That's obviously short of 100%, but given the recent push, unanimity appears inevitable.
It's worth noting that Tester is a "red"-state Democrat who was narrowly re-elected last year, as was Sen. Claire McCaskill (D-Mo.), who also endorsed marriage equality the other day. Some other recent announcements have come from "red"-state Democrats -- Virginia's Mark Warner and Alaska's Mark Begich -- who announced their support for same-sex marriage before their re-election fights in 2014.