Political party in the United States in the context of "Libertarian Party (United States)"

The Free Soil Party, also called the Free Democratic Party or the Free Democracy, was a political party in the United States from 1848 to 1854, when it merged into the Republican Party. The party was focused on opposing the expansion of slavery into the western territories of the United States. The 1848 presidential election took place in the aftermath of the Mexican–American War and debates over the extension of slavery into the Mexican Cession. After the Whig Party and the Democratic Party nominated presidential candidates who were unwilling to rule out the extension of slavery into the Mexican Cession, anti-slavery Democrats and Whigs joined with members of the Liberty Party (an abolitionist political party) to form the new Free Soil Party. Running as the Free Soil presidential candidate, former President Martin Van Buren won 10.1 percent of the popular vote, the strongest popular vote performance by a third party up to that point in U.S. history.

Though Van Buren and many other Free Soil supporters rejoined the Democrats or the Whigs after the 1848 election, Free Soilers retained a presence in Congress over the next six years. Led by Salmon P. Chase of Ohio, John P. Hale of New Hampshire, and Charles Sumner of Massachusetts, the Free Soilers strongly opposed the Compromise of 1850, which temporarily settled the issue of slavery in the Mexican Cession. Hale ran as the party's presidential candidate in the 1852 presidential election, taking just under five percent of the vote. The 1854 Kansas–Nebraska Act repealed the long-standing Missouri Compromise and outraged many Northerners, contributing to the collapse of the Whigs and spurring the creation of a new, broad-based anti-slavery Republican Party. Most Free Soilers joined the Republican Party, which emerged as the dominant political party in the United States in the Third Party System (1856–1894).

The Socialist Party of America (SPA) was a socialist political party in the United States formed in 1901 by a merger between the three-year-old Social Democratic Party of America and disaffected elements of the Socialist Labor Party of America who had split from the main organization in 1899.

In the first decades of the 20th century, the SPA drew significant support from many different groups, including trade unionists, progressive social reformers, populist farmers and immigrants. Eugene V. Debs twice won over 900,000 votes in presidential elections (1912 and 1920), while the party also elected two U.S. representatives (Victor L. Berger and Meyer London), dozens of state legislators, more than 100 mayors, and countless lesser officials. The party's staunch opposition to American involvement in World War I, although welcomed by many, also led to prominent defections, official repression, and vigilante persecution. The party was further shattered by a factional war over how to respond to the October Revolution in the Russian Republic in 1917 and the establishment of the Communist International in 1919—many members left the Socialist Party to found Leninist parties including the Communist Party USA.

The Green Party of the United States (GPUSA) is a federation of Green state political parties in the United States. The party promotes green politics, specifically environmentalism, nonviolence, social justice, participatory democracy, anti-war, and anti-racism. As of 2023, it is the fourth-largest political party in the United States by voter registration, behind the Libertarian Party. The Party first gained traction in 2000, when Ralph Nader ran for president.

The direct predecessor of the GPUSA was the Association of State Green Parties (ASGP). In the late 1990s, the ASGP, which formed in 1996, had increasingly distanced itself from the Greens/Green Party USA (G/GPUSA), America's then-primary green organization which had formed in 1991 out of the Green Committees of Correspondence, a collection of local green groups active since 1984. In 2001, the GPUS was officially founded as the ASGP split from the G/GPUSA. After its founding, the GPUS soon became the primary national green organization in the country, surpassing the G/GPUSA. John Rensenbrink and Howie Hawkins were co-founders of the Green Party.

The Socialist Labor Party (SLP) is a political party in the United States. It was established in 1876, and was the first socialist party formed in the country.

Descended from the Workingmen's Party of the United States, the party changed its name in 1877 to Socialistic Labor Party and again sometime in the late 1880s to Socialist Labor Party. The party was additionally known in some states as the Industrial Party or Industrial Government Party. In 1890, the SLP came under the influence of Daniel De Leon, who used his role as editor of The Weekly People, the SLP's English-language official organ, to expand the party's popularity beyond its then largely German-speaking membership. Despite his accomplishments, De Leon was a polarizing figure among the SLP's membership. In 1899, his opponents left the SLP and merged with the Social Democratic Party of America to form the Socialist Party of America.