Rep. Alexandria Ocasio-Cortez (D-NY) almost looks like a centrist now. That would have been a strange thing to say eight years ago, when she knocked off Rep. Joe Crowley, then the fourth-ranking Democrat in the House, and became the face of a new left flank that made party leaders nervous. She and the rest of the “Squad” were supposed to be the ceiling, the outer edge of what the activist base would tolerate, and Democratic leadership spent the next several years assuring anyone who would listen that the fever would break on its own, but it didn’t. If anything, the...