On May 30, 1854, the Kansas-Nebraska Act repealed the Missouri Compromise creating the territories of Kansas and Nebraska. From the article:
"Introduction of the Kansas-Nebraska Act in 1854
Known as the “Little Giant,” Douglas was one of the country’s most prominent politicians by 1854, and was seen as a likely future president. He was also a big booster of the planned transcontinental railroad, which would provide faster, more reliable transportation across the country. Douglas wanted the railroad to be built along a northern route that would go through Chicago as well as a vast area of land known as the Nebraska Territory, which had been included in the Louisiana Purchase.
Southern slaveholders and their allies in Congress opposed Douglas’ initial bill to organize the Nebraska Territory. In 1821, the Missouri Compromise had outlawed slavery everywhere in the remaining Louisiana Purchase lands north of the 36º 30’ parallel, and the two proposed territories lay north of this line.
Douglas needed proslavery votes to pass his “Nebraska Bill,” as it was known at the time. To get them, he added an amendment that repealed the Missouri Compromise and created two new territories, Kansas and Nebraska. Settlers in each territory would vote on the issue of whether to permit slavery or not, according to the principle of popular sovereignty."