Established in 1876 as the Workingmen's Party and renamed in 1877, the Socialist Labor Party (SLP) is the oldest socialist political party in the United States, and the second oldest socialist party in the world. Until 1889, the party was controlled by a conservative faction committed to a purely political program following the views of socialist theoretician Ferdinand Lassalle. In the 1890s, the Lassallean wing was overtaken by a more radical faction under the leadership of Daniel De Leon, transforming the SLP's platform into one focused on militant trade unionism and political action, and taking stands against the party's previous position of political reform and the trade union activity of the American Federation of Labor.

Beginning in 1900, the SLP saw a group of moderates, led by Morris Hillquit and known as the "Kangaroos," split from the party and advocate a return to a more conservative socialist program, eventually joining with the Social Democratic Party to establish the Socialist Party of America in 1901. This split, followed by De Leon's death in 1914, led to a swift decline in the SLP's activity, which became largely restricted to purely educational agitation, although the party would continue to field national tickets in every presidential campaign from 1892 through 1976 and publish the official newspaper "Weekly People" until the closure of its national office on September 1, 2008.