Democrats had a moment of unity after President Trump suggested four House freshmen “go back” to the “places from which they came”—but only a moment. Expect another progressive–centrist flare-up within days.

Progressives led by Bernie Sanders, Elizabeth Warren and Alexandria Ocasio-Cortez are in the fourth year of a campaign to seize the party from establishment corporatists like Nancy Pelosi and Chuck Schumer. Annoyed moderates, facing their first serious challenge since the New Left of the 1960s, have responded with backroom...