After President Obama made his switch on Marriage Equality last year everyone quickly realized that he'd be the last nominee of the Democratic party who hadn't endorsed marriage equality. His decision also made it possible for Democrats in blue states to make the move. But we've seen this avalanche of support now including Dems in states where marriage equality probably remains a net political loser, at least for now.
In some cases, this is just people doing what they believe is right. Good for them. Or in the case of Heidi Heitkamp who was just elected she probably realizes that this will be a settled issue in six years when she has to run again or if not, best to get it out of the way now regardless.
But there's one part of what's moving the dial that I've seen less discussion of: money. The word has clearly gone out that if you want to fund a Senate campaign in the Democratic Party, you have to get right on Marriage Equality. Period. If you're not, the funder class just will not be there for you, not to mention the grassroots Dems who give their $50 and $100 checks.
Again, I don't think this is a bad thing at all. But if you're looking to understand the avalanche that is a big part of what's driving it.