The Socialist Party of America (SPA) was a multi-tendency democratic-socialist political party in the United States, formed in 1901 by a merger between the three-year-old Social Democratic Party of America and disaffected elements of the Socialist Labor Party which had split from the main organization in 1899.
In the first decades of the 20th century, it drew significant support from many different groups, including trade unionists, progressive social reformers, populist farmers, and immigrant communities. Its presidential candidate, Eugene V. Debs, twice won over 900,000 votes (in 1912 and 1920), while the party also elected two United States Representatives (Victor L. Berger and Meyer London), dozens of state legislators, more than a hundred mayors, and countless lesser officials. The party's staunch opposition to American involvement in World War I, although welcomed by many, also led to prominent defections, official repression and vigilante persecution. The organization was further shattered by a factional war over how it should respond to the October Revolution in Russia in 1917 and the establishment of the Communist International in 1919.
After endorsing Robert LaFollette's presidential campaign in 1924, the Socialist Party returned to independent action. It had modest growth in the early 1930s behind presidential candidate Norman Thomas. While the party was always strongly anti-Fascist, as well as anti-Stalinist, the SPA's ambivalent attitude towards World War II cost it both internal and external support. After the 1950s, however, the Party's appeal was weakened by the popularity of President Franklin Roosevelt's New Deal, the organization and flexibility of the Communist Party under Earl Browder, and the resurgent labor movement's desire to support sympathetic Democratic Party politicians. A divisive and ultimately unsuccessful attempt to broaden the party by admitting followers of Leon Trotsky and Jay Lovestone caused the traditional "Old Guard" to leave and form the Social Democratic Federation.
The SPA stopped running presidential candidates after 1956, when its nominee Darlington Hoopes won fewer than 6,000 votes. In the party's last decades, its members, many of them prominent in the labor, peace, civil rights and civil liberties movements, fundamentally disagreed about the socialist movement's relationship to the labor movement and Democratic Party in the U.S., and about how best to advance democracy abroad. In 1970–1973, these strategic differences had become so acute that the Socialist Party changed its name to Social Democrats, USA. Leaders of two of its caucuses formed separate socialist organizations, the Democratic Socialist Organizing Committee and the Socialist Party USA.
This Socialist Party of America history is divided into the following periods: