1920s: Early in 1920, the Communist Party of the United States (CPUSA) has about 60,000 members. However, the “Red Scare” period, during which Woodrow Wilson's attorney general orders the arrest of some 10,000 suspected communists and anarchists, helps reduce the party's membership to 7,000 by 1929.

1960s: The Great Depression and the alliance of Russia and the United States in World War II gave the CPUSA membership a boost in the 1930s and 1940s, but this time it is the scare tactics of Senator Joseph McCarthy and the revelation of Stalin's crimes that diminish membership. By 1960 the party has around 10,000 members, down from an estimated peak of 100,000 during the war. The prosperity of the 1960s means that few are inspired to join the party, and though radicalism has increased in the late 1960s, the Communist Party does not play a significant role, and membership remains low.

1920s: During the...