On May 30, 1854, the Kansas-Nebraska Act repealed the Missouri Compromise creating the territories of Kansas and Nebraska. From the article:
"Kansas-Nebraska Act
The Kansas-Nebraska Act was an 1854 bill that mandated “popular sovereignty”–allowing settlers of a territory to decide whether slavery would be allowed within a new state’s borders. Proposed by Stephen A. Douglas–Abraham Lincoln’s opponent in the influential Lincoln-Douglas debates–the bill overturned the Missouri Compromise’s use of latitude as the boundary between slave and free territory. The conflicts that arose between pro-slavery and anti-slavery settlers in the aftermath of the act’s passage led to the period of violence known as Bleeding Kansas, and helped paved the way for the American Civil War (1861-65).
This 1854 bill to organize western territories became part of the political whirlwind of sectionalism and railroad building, splitting two major political parties and helping to create another, as well as worsening North-South relations.
On January 4, 1854, Stephen A. Douglas, wanting to ensure a northern transcontinental railroad route that would benefit his Illinois constituents, introduced a bill to organize the territory of Nebraska in order to bring the area under civil control. But southern senators objected; the region lay north of latitude 36°30′ and so under the terms of the Missouri Compromise of 1820 would become a free state. To gain the southerners’ support, Douglas proposed creating two territories in the area–Kansas and Nebraska–and repealing the Missouri Compromise line. The question of whether the territories would be slave or free would be left to the settlers under Douglas’s principle of popular sovereignty. Presumably, the more northern territory would oppose slavery while the more southern one would permit it.
Although initially concerned about the political fallout, President Franklin Pierce gave Douglas and his southern allies his support. The “Appeal of the Independent Democrats,” signed by such Free-Soilers as Salmon P. Chase and Charles Sumner and published in many northern newspapers, attacked Pierce, Douglas, and their supporters for breaking a sacred compact by repealing the Missouri Compromise.
The act passed Congress, but it failed in its purposes. By the time Kansas was admitted to statehood in 1861 after an internal civil war, southern states had begun to secede from the Union. The Independent Democrats and many northern Whigs abandoned their affiliations for the new antislavery Republican party, leaving southern Whigs without party links and creating an issue over which the already deeply divided Democrats would split even more. The railroad was eventually built but not along the route Douglas wanted and with funds voted by a Republican Congress during a Republican Civil War administration."