A serious argument over slavery took place in Congress when Missouri asked to be admitted to the Union in 1819. Missouri was a territory making up part of the Louisiana Purchase. Many settlers in Missouri came from southern states. Slavery was allowed in Missouri.
At that time, there were exactly the same number of "free" states (states which did not allow slavery) as there were "slave" states (states which did allow slavery). This meant that there was a balance of power in the United States Senate, where all states are represented equally. The "free" states had a majority in the House of Representatives because the northern states had a larger population than the southern states.
When Missouri's request to be admitted reached Congress, a northern Congressmen proposed that it should be admitted only if it promised to free its slaves. Southerners in Congress angrily replied that the North had no right to require this.
In December 1819, Maine also asked to be admitted to the Union. The House of Representatives was ready to approve Maine's request. There was no slavery in Maine. However, at first the Senate would not allow Maine to enter the Union. Southern senators refused to admit Maine as a free state unless the northern senators would agree to admit Missouri as a slave state.
In 1820 Henry Clay of Kentucky helped arrange a compromise that allowed both Maine and Missouri to enter the Union. By means of the Missouri Compromise, as the new agreement was called, several laws were passed. Congress admitted Maine as a free state and Missouri as a slave state. Slavery had now spread to one more state. At the same time, Congress drew a line westward across what was left of the Louisiana Purchase. This line, set at 36° 30' North latitude, divided free and slave territory. In the future, any states formed north of the line would be free states. Any new states formed south of the line would be open to slavery.