Conference Committee Report on the Missouri Compromise, March 1, 1820
When Missouri, which allowed slavery, applied for statehood in 1819 Congress struggled for a way to maintain the Union despite strongly opposing pro- and anti-slavery constituencies. After heated debate, Congress adopted the Missouri Compromise, which admitted Maine as a free state to balance Missouri and prohibited slavery north of the 36º 30´ latitude in the Louisiana Territory.
…in all that Territory…under the name of Louisiana, which lies north of thirty six degrees and thirty minutes north latitude…Slavery and involuntary Servitude…shall be and is hereby forever prohibited.
Records of Joint Committees of Congress, National Archives and Records Administration, Washington, D.C.
Preserving the Union - 1
To achieve unity among the states, the nation’s founders did not confront the issue of slavery in the Constitution, but left it for future generations to resolve. In the 19th century, applications for statehood from western territorial legislatures threatened the balance of free and slave states in the Senate. Congress shifted the basis for determining the status of slavery in each territory from the territory’s geographical location to a vote by its inhabitants under the principle of ‘popular sovereignty.’ Slavery remained a burning issue that threatened the Union and eventually led to the Civil War.