Communist Party of the United States of America (CPUSA), left-wing political party in the United States that was, from its founding in 1919 until the latter part of the 1950s, one of the country’s most important leftist organizations. Its membership reached its peak of 85,000 in 1942, just as America entered World War II; the CPUSA had rallied enthusiastically in favour of a Soviet-American war effort against Nazi Germany.

In 1919, inspired by Russia’s October Revolution (1917), two U.S. communist parties emerged from the left wing of the Socialist Party of America (SPA): the Communist Party of America (CPA), composed of the SPA’s foreign-language federations and led by the sizeable and influential Russian Federation, and the Communist Labor Party of America (CLP), the predominantly English-language group. They were established legally but were soon forced underground. Although the two parties feuded and various factions broke away to establish competing communist groups, the Communist International encouraged the unification of those organizations. In 1922 the CPA merged with the United Communist Party (which had been established when the CLP joined a breakaway faction of the CPA) to create the legal and aboveground Workers Party of America (WPA). When the United Toilers of America, a group that adopted the same tactics as the WPA, combined with the latter organization, the party renamed itself the Workers (Communist) Party, finally settling on the name Communist Party of the United States of America in 1929.

During the 1920s the CPUSA’s trade-union arm, the Trade Union Educational League, promoted industrial unionism vis-à-vis the craft union-oriented American Federation of Labor (AFL). When that strategy proved unsuccessful, the CPUSA upon orders from Moscow transformed the Trade Union Educational League into the Trade Union Unity League in 1929, which was dedicated to organizing largely unskilled immigrant, African American, and female workers into industrial unions. Although the Trade Union Unity League was not nearly as successful as the AFL, it did provide a training ground for CPUSA organizers when they became active in the Congress of Industrial Organizations (CIO) unions.

During the early years of the Great Depression, the CPUSA emerged as committed militants within the unemployed movement. Later in the 1930s, with approximately 65,000 members and New Deal liberalism sweeping the country, the CPUSA became influential in many aspects of life in the United States. There were also untold numbers of “fellow travelers” who sympathized with the aims of the party though they never became members of it. At that time CPUSA members became national, regional, and community leaders in liberal, cultural, and student organizations. In addition, because of their roles as industrial union organizers during the mid-to-late 1930s, they became a major force in several important CIO unions by the early 1940s. In New York City, a stronghold of party support where communists actively engaged in housing struggles, CPUSA candidates were elected to the city council during its zenith.

After World War II, with the onset of the Cold War and the rise of anti-Soviet sentiment, the CPUSA increasingly came under attack. Deprived of significant influence in the labour movement when the CIO expelled 11 CPUSA-led unions in 1949 and 1950, the CPUSA suffered additional losses of power in many left-liberal organizations when it was subjected to McCarthyism in the early 1950s. In 1956 support for the Soviet invasion of Hungary and the revelation of Joseph Stalin’s crimes in Nikita Khrushchev’s “secret speech” at the 20th Soviet Party Congress led to mass defections from the CPUSA. Although communists held leadership positions in several anti-Vietnam War organizations during the 1960s and ’70s, they exerted little sway in the U.S. labour movement. While the party made many significant contributions to the radical movement, especially during the 1930s and ’40s, the CPUSA’s unswerving support for Stalin and the Soviet Union harmed the party not only in the eyes of broad segments of the population but among other liberal and left-wing activists as well.