Speaker Nancy Pelosi this week stepped up to stare down a rebellion, and the Democrats’ 2020 chances may hinge on whether she proves successful. Watch to see if the steely-eyed Californian can convince her party’s pragmatists to hold out against its prima donnas.

Mrs. Pelosi is taking heat for a Wednesday caucus meeting at which she scolded Democrats for publicly attacking each other. “You got a complaint? You come and talk to me about it. But do not tweet about our members and expect us to think that that is just OK,” Mrs....