Trump outdid himself with anti-“squad” tweets over the weekend suggesting that the four Democratic congresswomen go back where they came from. The tweets were clearly trolling and meant to be sardonic — supposedly they are to go back home, fix their countries, and then return here to tell us how it’s done, with Nancy Pelosi happy to pay the airfare. They are several things to say about this: 1) As always, presidents shouldn’t troll; 2) on a tactical level, it’s incredibly dumb for the president to insert himself into the middle of the Democratic circular firing squad between Nancy Pelosi...