The surprise electoral success of Zohran Mamdani, the 33-year-old democratic socialist running to be mayor of New York, most prominent city on Earth, is a political earthquake. The breadth and scope of his performance were predicted by no polls, no prognosticators, none of the wise men. The ramifications of this upset will be felt for years, across the US and the developed world. In the end, it wasn’t even close. Mamdani’s widespread appeal represents the total collapse of a Democratic party establishment that had weathered Donald Trump’s first term with rhetorical resistance, and fumbled the beginning of the second with...