by Patrick Appel
Nate Silver expects it to slow down:
[I]f the recent cavalcade of endorsements is caused in part by senators perceiving that same-sex marriage has potentially become the national majority position, endorsements will begin to decelerate once it has become unambiguously the majority stance. Some senators will continue to oppose it, either because it does not yet constitute a majority position in their states (like Senator Heidi Heitkamp of North Dakota, they may say it should be decided at the state level), or because they oppose it on moral grounds, or because they are more concerned about a primary challenge than the general election.
In other words, the past year or two has been a good time for senators to jump on the same-sex marriage bandwagon, and most of the stragglers (i.e., Democrats from blue or purple states) have been rounded up. The remaining senators who have not taken the opportunity yet may have good political reasons for it, and may wait some time before they do.
Meanwhile, National Journal names the five Republican senators most likely evolve on marriage equality.