The Kansas–Nebraska Act of 1854 created the territories of Kansas and Nebraska, opening new lands for settlement, and had the effect of repealing the Missouri Compromise of 1820 by allowing settlers in those territories to determine through Popular Sovereignty whether they would allow slavery within each territory. The act was designed by Democratic Senator Stephen A. Douglas of Illinois. The initial purpose of the Kansas–Nebraska Act was to open up many thousands of new farms and make feasible a Midwestern Transcontinental Railroad. It became problematic when popular sovereignty was written into the proposal so that the voters of the moment would decide whether slavery would be allowed. The result was that pro- and anti-slavery elements flooded into Kansas with the goal of voting slavery up or down, leading to a bloody civil war there.

Douglas hoped popular sovereignty would enable democracy to triumph, so he would not have to take a side on the issue of slavery. A wave of indignation erupted across the North as anti-slavery elements cried betrayal, for Kansas had been officially closed to slavery since the Missouri Compromise of 1820, and that Compromise was now repealed because of popular sovereignty. Opponents denounced the law as a triumph of the hated slave power — that is the political power of the rich slave owners, who would buy up the best lands in Kansas leaving ordinary men with the leftovers. The new Republican Party, which was created in opposition to the act, aimed to stop the expansion of slavery and soon emerged as the dominant political party in the North, electing its first president, Abraham Lincoln, in 1860.