Third party (U.S. politics) in the context of "Two-party system"

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 The 1854 Kansas–Nebraska Act repealed the long-standing Missouri Compromise and outraged many Northerners, contributing to the collapse of the Whigs and spurring the creation of a new, broad-based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 Liberty Party was an abolitionist political party in the United States before the American Civil War. The party experienced its greatest activity during the 1840s, while remnants persisted as late as 1860. It supported James G. Birney in the presidential elections of 1840 and 1844. Others who attained prominence as leaders of the Liberty Party included Gerrit Smith, Salmon P. Chase, Henry Highland Garnet, Henry Bibb, and William Goodell. They attempted to work within the federal system created by the United States Constitution to diminish the political influence of the Slave Power and advance the cause of universal emancipation and an integrated, egalitarian society.

In the late 1830s, the antislavery movement in the United States was divided between Garrisonian abolitionists, who advocated nonresistance and anti-clericalism and opposed any involvement in electoral politics, and Anti-Garrisonians, who increasingly argued for the necessity of direct political action and the formation of an anti-slavery third party. At a meeting of the American Anti-Slavery Society in May 1840, the Anti-Garrisonians broke away from the Old Organization to form the American and Foreign Anti-Slavery Society. The New Organization included many political abolitionists who gathered in upstate New York to organize the Liberty Party ahead of the 1840 elections. They rejected the Garrisonian singular emphasis on moral suasion and asserted that abolitionists should oppose slavery by all available means, including by coordinating at the ballot box.

The Progressive Party, popularly nicknamed the Bull Moose Party, was a third party in the United States formed in 1912 by former president Theodore Roosevelt after he lost the presidential nomination of the Republican Party to his former protégé turned rival, incumbent president William Howard Taft. The new party was known for taking advanced positions on progressive reforms and attracting leading national reformers. The party was also ideologically deeply connected with America's radical-liberal tradition. After the party's defeat in the 1912 United States presidential election, it went into rapid decline in elections until 1918, disappearing by 1920. The "Bull Moose" nickname originated when Roosevelt boasted that he felt "strong as a bull moose" after losing the Republican nomination in June 1912 at the Chicago convention.

As a member of the Republican Party, Roosevelt had served as president from 1901 to 1909, becoming increasingly progressive in the later years of his presidency. In the 1908 presidential election, Roosevelt helped ensure that he would be succeeded by Secretary of War Taft. Although Taft entered office determined to advance Roosevelt's Square Deal domestic agenda, he stumbled badly during the Payne–Aldrich Tariff Act debate and the Pinchot–Ballinger controversy. The political fallout of these events divided the Republican Party and alienated Roosevelt from his former friend. Progressive Republican leader Robert M. La Follette had already announced a challenge to Taft for the 1912 Republican nomination, but many of his supporters shifted to Roosevelt after the former president decided to seek a third presidential term, which was permissible under the Constitution prior to the ratification of the Twenty-second Amendment. At the 1912 Republican National Convention, Taft narrowly defeated Roosevelt for the party's presidential nomination. After the convention, Roosevelt, Frank Munsey, George Walbridge Perkins and other progressive Republicans established the Progressive Party and nominated a ticket of Roosevelt and Hiram Johnson of California at the 1912 Progressive National Convention. The new party attracted several Republican officeholders, although nearly all of them remained loyal to the Republican Party—in California, Johnson and the Progressives took control of the Republican Party.