The Socialist Party of America was established in 1901. By that time, anarchism also established itself around the country while socialists of different tendencies were involved in early American labor organizations and struggles which reached a high point in the Haymarket affair in Chicago which started International Workers' Day as the main workers holiday around the world (except in the United States, which celebrates Labor Day on the first Monday of September) and making the 8-hour day a worldwide objective by workers organizations and socialist parties worldwide.

Under Socialist Party of America presidential candidate Eugene V. Debs, socialist opposition to World War I led to the governmental repression collectively known as the First Red Scare. The Socialist Party declined in the 1920s, but nonetheless often ran Norman Thomas for President. In the 1930s, the Communist Party USA took importance in labor and racial struggles while it suffered a split which converged in the Trotskyist Socialist Workers Party. In the 1950s, socialism was affected by McCarthyism and in the 1960s it was revived by the general radicalization brought by the New Left and other social struggles and revolts. In the 1960s, Michael Harrington and other socialists were called to assist the Kennedy administration and then the Johnson administration's War on Poverty and Great Society while socialists also played important roles in the Civil Rights Movement. Socialism in the United States has been composed of many tendencies, often in important disagreements with each other; it has included utopian socialists, social democrats, democratic socialists, communists, Trotskyists and anarchists.

The socialist movement in the United States has historically been relatively weak. Unlike socialist parties in Europe, Canada and Oceania, a major social democratic party never materialized in the United States and the socialist movement remains marginal, "almost unique in its powerlessness among the Western democracies". In the United States, socialism "brings considerable stigma, in large part for its association with authoritarian communist regimes"