The Socialist Party of America was founded in 1901, from a fusion of the Social Democratic Party and the so-called “Rochester” faction of the Socialist Labor Party. For a time the Socialist Party gained membership and electoral strength rapidly, reaching a pinnacle in 1912 of nearly one million votes or 16 percent of the electorate. Internal disputes, however, prevented the Party from advancing further. In 1913, approximately fifteen percent of the Socialist Party's members left its ranks over the issue of sabotage and syndicalism generally. In 1919, Socialist Party leaders expelled a majority of the Party's members, chiefly those who identified themselves with the leaders of the Russian Revolution. By 1921 the internal struggle, along with government repression, reduced the Socialist Party to a slight fraction of its former size.
The Socialist Party never fully recovered from the splits of 1919 to 1921. In the 1920s it assumed a position to the right of the Communist Party and accepted the leadership of Norman Thomas. In the 1930s, international and national pressures threw the Socialist Party Right against its Left, resulting finally in the withdrawal of the extreme moderates from the Party in 1936 to form the Social Democratic Federation. In 1938 the extreme Left wing was expelled from the Party, and moved toward the formation of the Socialist Workers Party.
In the 1940s and 1950s the Socialist Party almost fully accepted the premises of American foreign policy toward the Soviet Union and drew increasingly closer to orthodox American liberalism. In 1957, the Party merged with the Social Democratic Federation, and in 1958 accepted the membership of the Independent Socialist League. But despite unity, the Socialist Party-Social Democratic Federation (as it was renamed in 1958) continued to shrink in size and influence. By the 1960s the Socialist Party, retaining only a few well-known spokesmen, such as Norman Thomas and Erich Fromm, was scarcely more than an appendage to the liberal wing of the Democratic Party. It rarely contested elections, its press was reduced to the monthly newspaper, New America, and its membership was no more than a few thousand.
