IN THE LATE '40S, AMERICAN LIBERALISM MADE A FATEFUL decision: it went into the business of excommunication. Liberalism's leading lights -- figures like Joseph Rauh, Walter Reuther and Hubert Humphrey -- understood that unless they clearly separated themselves from communists and their fellow travelers they risked losing not just their souls but their political viability. Hence Principle 6 of the founding statement of Americans for Democratic Action: "We reject any association with Communists or sympathizers with communism."
In America, political movements need to police their extremes. Conservatism is no exception. It is a matter not just of principle, but of practical...