The Socialist Labor Party of the United States was the nation's oldest socialist political party until it became inactive in 2008. The party was created in 1876 as the Workingmen's Party and was renamed in 1877. The party originally attracted a variety of socialist members but, by 1890, the party had adopted a Marxist world view.

Remaining party members closed the group's national office on 1 September 2008 and sent most of its materials to be archived at Duke University and the Wisconsin State Historical Society. The party began existing entirely on paper in 2008, with hopes of someday reviving its financial situation. All local and state party offices closed.

The Socialist Labor Party was very active in American politics through the mid-1970s, fielding a candidate in every presidential election from 1892 to 1976. The party was organized in local sections with a bi-annual national convention of democratically elected delegates. Membership was open to anyone who agreed with the party's program, agreed to abide by its tenets, and had no ties to other political parties.

The party's political agenda, called the Socialist Industrial Union Program, sought to create a classless society by developing a Marxist class consciousness among workers across industries. All property would be collectively owned, with industries and social services operated for the benefit of all and supervised by the democratically elected Socialist Industrial Union government. The agenda for achieving this goal followed Marx in focusing on the class consciousness of the working class. The thought was that, by developing a collective consciousness, the working class could develop the political and economic institutions that would allow it to seize power from the controlling capitalist class.

The early days of the Socialist Labor Party were filled with attempts to create the necessary political and economic institutions to accomplish its mission. The party tried to turn the American Federation of Labor and other unions into militant working-class organizations in the late 19th century, but the effort eventually failed. It endorsed the Socialist Trade & Labor Alliance and continued to support it until 1905, when the alliance merged with the Industrial Workers of the World.

The Socialist Labor Party then helped to establish a new International Workers of the World when the original organization was overtaken by anarchists in 1908. Based in Detroit, the new International Workers of the World was later renamed the Workers International Industrial Union. It operated under that name until 1924, when it was disbanded. After that year, the Socialist Labor Party was not involved directly in establishing outside organizations.

The party highlighted the contributions of one of its early members, Daniel De Leon. De Leon developed the Socialist Industrial Union Program and served as editor of The People, the party's political magazine, from 1982 until his death in 1914. While recognizing De Leon's contributions, the Socialist Labor Party was careful to emphasize that he was only one of many great socialists and that the party should not be thought of as a cult of personality.