Add two more senators to the list of gay-marriage "evolvers"—North Dakota's Heidi Heitkamp and Indiana's Joe Donnelly said today that they support same-sex marriage, reports the Huffington Post. Both are Democrats from conservative states and both are safe from re-election campaigns until 2018. "The makeup of families is changing, but the importance of family is enduring," said Heitkamp in a statement. Together with yesterday's flip by Florida's Sen. Bill Nelson, the moves leave only four Democratic senators who have not publicly voiced support for gay marriage.
If you're a bit amazed by how quickly senators have been "evolving" on this issue, you're not the only one. But Nate Silver decided to take things a step further, as usual, and try to come up with a mathematical model to explain why, for his latest blog post in the New York Times. The key, says Silver, is public support for gay marriage in many states growing from a slight majority to a clear one. But he expects the pace to slow down greatly at this point. "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," he writes. (Read more Heidi Heitkamp stories.)