In the leadup to Thomas Massie’s primary defeat last week, I was struck by how popular the Kentucky congressman had suddenly become among people who don’t vote in Republican primaries—indeed, among people who don’t like Republicans, don’t like Kentucky, and until the day before yesterday never liked Thomas Massie. This, I thought, didn’t augur well for the congressman’s reelection, which would depend on actual Republicans voting in the Republican primary. The results were worse than I would have guessed, however: a 10-point blowout. Massie’s explanation for his sunken popularity with the voters of his own party and his own district...