In the late 1950s, Bill Buckley decreed that nobody whose name appeared on the masthead of the American Mercury magazine would be published in the pages of National Review. The once-illustrious Mercury of H.L. Mencken had become a gutter of far-right anti-Semites. Buckley would not allow his magazine to be tainted by them.

The word for Buckley’s act is “lustration,” and for two generations it upheld the honor of the mainstream conservative movement. Liberals may have been fond of claiming that Republicans were all closet bigots...