In 1854, the Missouri Compromise of 1820 was superseded by the Kansas–Nebraska Act. This allowed white male settlers in the new territories to determine through 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. This led to the conflict known as Bleeding Kansas. An effort was attempted to organize Kansas for admission as a slave state. It was to be paired with Minnesota as a free state. But the admission of Kansas as a slave state was blocked because of questions over the legitimacy of its slave state constitution. Anti-slavery settlers in Kansas in the 1850s were called "Free-Soilers", because they fought (successfully) to include Kansas in the Union as a free state in 1861. When the admission of Minnesota proceeded unimpeded in 1858, the balance in the Senate was lost. Oregon was admitted in 1859 as a free state.