So far, Donald Trump is having the worst campaign launch since Beto O’Rourke. Like almost everything else he’s done lately, his early announcement of his next presidential campaign has proved a flagrant political misjudgment. A move that was supposed to demonstrate his strength is showing his weakness; a move that was meant to keep other candidates out of the race is an invitation to other candidates to get in; a move that was supposed to serve notice of his continued dominance of the party is pointing toward its potential end. He’s been eclipsed as an Internet troll by Elon Musk,...