Modern liberalism has broken from its roots by focusing on groups rather than individuals, reimagining Western history as irredeemably corrupt, and celebrating victimhood. The danger is not only that the new left might win in the marketplace of ideas, but that conservatives will pervert their own beliefs in response.

Not long ago, liberals and conservatives alike celebrated and fought for universal individual rights, as opposed to a world in which individuals are defined primarily by their ethnicity, sex and sexuality. Martin...