Joe Biden refuses to drop out of the presidential race even as some liberals, rattled by the incumbent’s frightening debate performance last week, keep pressuring him to do so.Who’s surprised by that?The inertia of a presidential campaign is one of the most powerful forces in politics. Ending one after a party’s nomination has been secured is almost unfathomable. The candidate is already strapped to the rocket.Furthermore, all serious presidential contenders, particularly those who hold or have already held the office — this year, we have both — have a God complex. They must. And doubt doesn’t exist in the presence...