In 1854, the Missouri Compromise of 1820 was superseded by the Kansas–Nebraska Act, which allowed white male settlers in the new territories to determine, by vote (popular sovereignty), whether they would allow slavery within each territory. The result was that pro- and anti-slavery elements flooded into Kansas with the goal of voting slavery up or down, leading to bloody fighting. An effort was initiated to organize Kansas for admission as a slave state, paired with Minnesota, but the admission of Kansas as a slave state was blocked because its proposed pro-slavery constitution (the Lecompton Constitution) had not been approved in an honest election. Anti-slavery proponents during the "Bleeding Kansas" period of the later 1850s were called "Free-Staters" and "Free-Soilers", and fought against pro-slavery Border Ruffians from Missouri. The animosity escalated throughout the 1850s, culminating in numerous skirmishes and devastation on both sides of the question. Nevertheless, the North prevented Kansas Territory from becoming a slave state, and when Southern members of Congress departed "en masse" in early 1861, Kansas was immediately admitted to the Union as a free state.