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The formation of the Republican Party in the United States in the mid-1850s was a direct response to the growing controversy over the expansion of slavery into the newly acquired territories from the Mexican-American War and the dissatisfaction with the Kansas-Nebraska Act of 1854. Specifically, this act, created by Senator Stephen A. Douglas, allowed the people of the Kansas and Nebraska territories to decide for themselves whether or not to allow slavery within their borders, a concept known as "popular sovereignty." This effectively repealed the Missouri Compromise, which had prohibited slavery in that part of the Louisiana Purchase north of latitude 36°30'. The Kansas-Nebraska Act thus reopened the bitter debate over the spread of slavery, which many in the North had thought to be settled.
Opponents of the Kansas-Nebraska Act and the extension of slavery into the territories came from existing political parties, including Northern Democrats, Whigs, Free Soilers, and abolitionist factions. They were deeply concerned with the moral issue of slavery, as well as the political implications of its expansion, fearing it would give the slaveholding South too much power. These groups coalesced to form the Republican Party, putting forth a platform that sought to prevent the spread of slavery into any U.S. territories, though not to interfere with it where it already existed. The new party gained significant support in the North, leading to the election of the first Republican president, Abraham Lincoln, in 1860, which in turn prompted the secession of Southern states and the start of the Civil War.