Americans are moving to other states as they sort themselves by ideology. It’s a worrying trend.

The “Harry Potter” books famously feature a sorting hat, a magical way of determining in which house Hogwarts students truly belong: Gryffindor? Slytherin? Last week, The Associated Press reported that Americans have no need for such sorcery to find the like-minded: Republicans and Democrats are separating physically at such a furious pace, the news agency reported, the ideological divide between the states is now starker than at any point in living memory. The most striking evidence? A single party controls the legislature in all but two states. And only 10 states are led by governors of part...