The Socialist Labor Party, founded in 1876, in New York City is the oldest socialist party in the United States. The SPL was arguably at its most powerful in late 1870s when an SPL state senator was elected in Illinois as well as four Chicago city councilmen. Through the late 19th and early 20th Century the SPL enjoyed some success at the helm of Daniel De Leon even receiving 80,000 votes in the 1896 presidential election with Charles Matchett running. Ideological the SPL falls under “De Leonism” which is a form of Marxism that predates Leninism and posits that workers must simultaneously form socialist industrial unions in the workplaces, and a socialist political party in order to enact change.
The SPL also saw a slight bump in interest throughout the late 1950s and early 1960s under the leadership of Eric Hass but this did not have a lasting impact. Although the organization was not a powerful political party throughout the 20th Century it continued to publish its newspaper Weekly People and only closed its national office on September 1, 2008. There are ~350 items from the SPL in the Hall Hoag Collection Part II.