Stephen Douglas' most prominent belief was that popular sovereignty should determine the status of slavery in new U.S. territories. Before Douglas, Congress determined an area's designation as slave or free based on geography.
In the Missouri Compromise of 1820, the U.S Congress allowed Missouri to enter the union as a slave state but prevented any other territory north of 36°30' from allowing slaves. Although this helped maintain sectional balance in the short term, the southern states felt hemmed in by the ban and wanted to expand slavery. Stephen Douglas, first as a representative and then as U.S. Senator from Illinois, worked to change this ban. He argued consistently and eloquently for popular sovereignty, even debating Abraham Lincoln in the famous Lincoln-Douglas debates. Under this regime, residents of a territory would vote on whether they wanted to allow slavery.
He wrote and passed the Kansas-Nebraska Act of 1854, overturning the Missouri Compromise by allowing residents of territories north of the dividing line of latitude to vote on whether they wanted to allow slavery. Though intended to help relieve sectional tension by creating democratic legitimacy for a territory's status, this law actually helped increase conflict. Pro-slavery and anti-slavery factions emerged in Kansas, engaging in a series of violent engagements that became known as "Bleeding Kansas." A new party, the Republican Party, formed to oppose the extension of slavery. Douglas was sympathetic to the slave states, but his primary allegiance remained to the union: at Lincoln's inauguration, Douglas held the new president's hat.