Minor but influential 19th-century U.S. political party that opposed the extension of slavery into the western territories.
In 1846 proponents of the Wilmot Proviso and other antislavery factions formed a party; in 1848 it nominated former president Martin Van Buren to head its ticket. Though Van Buren lost, many party supporters were elected to the U.S. House of Representatives. By 1854 the party was absorbed into the Republican Party .