Free-Soil Party, American political party organized in 1848 on a platform opposing the extension of slavery. The growing conflict between proslavery and antislavery forces in the United States was intensified by the acquisition of new territories from Mexico and the ensuing argument over whether or not slavery would be permitted in those territories. The defeat of the Wilmot Proviso, which was intended to prevent the extension of slavery, and the struggle over it in Congress brought the conflict to a head; the refusal of both the Whig and Democratic parties to endorse the principles of the proviso convinced opposition groups of the need for a new party. The major groups involved in the organization of the Free-Soil Party at a convention in Buffalo, New York, in 1848 were the abolitionist Liberty Party, the antislavery Whigs, and a radical faction of the New York Democrats, the Barnburners, who had broken with the state party when it came under control of the conservative Hunkers.

The Free-Soil convention nominated Martin Van Buren and Charles Francis Adams as candidates for president and vice president, respectively, and adopted a platform opposed to the extension of slavery and calling also for a homestead law and a tariff for revenue only. The slogan of the party was “free soil, free speech, free labor, and free men.” The party polled 291,263 votes in the election of 1848; it carried no states, but turned the election in New York to the Whigs, and thus played a decisive role in the election of President Zachary Taylor. The party also elected 2 U.S. senators and 14 representatives. The Compromise Measures of 1850 on the extension of slavery caused the return of the Barnburners to the Democratic Party and the loss of other allies, but the Free-Soil Party continued to function; in 1852, even though it polled fewer votes than four years previously, it increased its representation in Congress. The passage of the Kansas-Nebraska Act in 1854 caused the final breaking of the old party lines and resulted in the formation of the Republican Party, into which the Free-Soil Party was absorbed.