There are few intellectual movements in modern American political history more successful than the Federalist Society. Created in 1982 to counterbalance what its founders considered a liberal legal establishment, the organization gradually evolved into the conservative legal establishment, and membership is all but required for any conservative lawyer who hopes to enter politics or the judiciary. It claims 40,000 members, including four Supreme Court Justices, dozens of federal judges, and every Republican attorney general since its inception. But its power goes even deeper.

In Ideas...