The slow decline of this ideological overlap has led inexorably to gridlock and dysfunction when one party controls the presidency and the other leads at least one chamber of Congress. There is is simply less to talk about. It’s not like disagreeing about whether to get a Border Collie or a Boston Terrier; it’s like if you want a dog and only a dog and nothing but a dog and your partner despises animals of all kinds.

But you would think that this sorting would make for more coherent ideological blocs more capable of making policy when one party controls Congress and the presidency, as Republicans do now. That was surely what Republican voters expected when they woke up triumphant on Nov. 9 last year. But the divide within the Republican Party is proving to be as problematic as polarization between the parties. The ideological distance between the Senate’s most liberal member (Maine’s Susan Collins) and the most hard-right senator (Utah’s Mike Lee) is the same as the chasm between a middle-of-the-pack Democrat like Maryland’s Ben Cardin and a conservative like Iowa’s Joni Ernst.

If you want to understand how much harder it is going to be for Republicans to get anything done than it was for the Democrats in 2009-2011, your best bet is to look at this intra-Republican distance.