Two recent events, on opposite sides of the Atlantic, point to the same problem. In Britain, the man widely expected to replace the stately Keir Starmer as prime minister is Andy Burnham, who touts “business-friendly socialism” as his credo. In New York, Democratic primary elections produced striking victories for democratic socialists, suggesting that the insurgent left has found a way to turn protest into power.

First, a caveat: The left is not marching uniformly toward socialism. Many primaries outside New York City were won by moderate Democrats. In a swing district just outside the city, combat veteran Cait Conley won handily. But a certain kind of liberalism is losing energy, confidence, and connection to the people it claims to represent.

Two recent events, on opposite sides of the Atlantic, point to the same problem. In Britain, the man widely expected to replace the stately Keir Starmer as prime minister is Andy Burnham, who touts “business-friendly socialism” as his credo. In New York, Democratic primary elections produced striking victories for democratic socialists, suggesting that the insurgent left has found a way to turn protest into power.

First, a caveat: The left is not marching uniformly toward socialism. Many primaries outside New York City were won by moderate Democrats. In a swing district just outside the city, combat veteran Cait Conley won handily. But a certain kind of liberalism is losing energy, confidence, and connection to the people it claims to represent.

In Adrian Wooldridge’s new book “The Revolutionary Center,” a brilliant intellectual history of liberalism from the Enlightenment to the present, Wooldridge reminds us that liberalism was once the most radical force in politics. It attacked inherited privilege, monopoly power, censorship, aristocracy, clerical authority, and closed guilds. It was not the ideology of the establishment. It was........

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