In a few days the U.S. will have its midterm results, and the Beltway press corps will lecture us on the lessons. Don’t expect to hear much about the one takeaway that is already obvious: that today’s preferred progressive politics—of character assassination, mob rule, intimidation and wacky policies—is an electoral bust. It is not what is winning Democrats anything. It is what is losing the party the bigger prize.

Six weeks ago, Democrats were expecting a blue wave to rival the Republican victory of 2010, when the GOP picked...