The Socialist Party of the United States is a major center-left political party in the United States and is one of the two main political parties, along with the center-right Democratic Party. The Socialist Party has its roots in the late 19th century labor movement, Progressive-era Republicans such as Teddy Roosevelt, and the agrarian Populist revolution in the 1890s and later 1910s. The Populist-backed People's Party merged with the American Socialist Party in 1915 in opposition to World War I after running a joint ticket backing William J. Bryan in 1908, and elected its first President in 1920 when it ran a joint ticket with the Liberal Republicans to nominate Robert M. LaFollette.
The Socialist Party, though having its roots in Marxist nationalization and anti-capitalism, has moved to more mainstream social democratic positions since the Presidency of Norman Thomas (1929-1933) and then particularly under Joseph P. Kennedy, leading to the term "Kennedian Socialism" to refer to the ideology of the party's right-to-center wing, in opposition to the more ardently left-wing "Reuther Socialists" with their roots deep in the labor movement.
The Socialist Party enjoyed the apex of its success from 1945-1961, when it controlled the White House for sixteen consecutive years under Presidents Joseph Kennedy and Adlai Stevenson, and controlled both houses of Congress uninterrupted from 1945-1975. During the Presidency of Hubert Humphrey, more militant labor unions, two steep recessions and the 1974 oil embargo soured public opinion on the dominant Socialists, sweeping Democrats to power in both houses of Congress in the 1974 midterms. The Socialists would narrowly hang on to power in 1976 with moderate Henry Jackson's win over arch-conservative California Governor Ronald Reagan, but were ousted from power in 1980 against the moderate George Bush, leaving Democrats in control of the entire government for the first time since the 1940s. The Socialists would retake the House in 1982 and the Senate in 1986, but it would not be until the "New Socialist" Mario Cuomo ran in 1992 that they would retake the White House. Since the 1980 landslide loss, the only Socialist Presidents have been Mario Cuomo (1993-2001) and his son, Andrew (2013-present). After losing both Houses of Congress again in 1994, the Socialists would retake the House in 2006 and the Senate in 2010, and presently control the full government for the first time since 1994.