By examining the Missouri Compromise, what can one learn about slavery as a political issue in the United States during the early nineteenth century?

Slavery had been a national issue prior to the Missouri Compromise, but it reached a boiling point in 1819. In 1819, there were 22 states in the Union: 11 free states and 11 slave states. This fact was important because it maintained a balance of power in Congress. Free states and slave states were equally represented in Congress, and the US sought to maintain that balance. However, that year Missouri requested to be admitted to the Union as a slave state, meaning there would be more slave states, tipping the balance in Congress. Debate broke out in Congress between representatives from free and slave states over whether or not to admit Missouri as a slave state. Many Northerners fiercely opposed Missouri's admission as a slave state, arguing that Congress had the authority to outlaw slavery in Missouri. On the other hand, many Southerners argued that states had the freedom to choose whether or not to allow slavery. Senator Henry Clay, known for his ability to compromise, proposed a solution: Missouri would be admitted as a slave state, and Maine, which had been part of Massachusetts, would be admitted as a separate free state, thus maintaining the balance between free and slave states. In addition, Clay proposed that slavery would henceforth be banned in the Louisiana Territory that was North of the 36' 30' latitude (except for Missouri). The compromise was not perfect. Southerners opposed the idea that Congress could ban slavery in any state or territory, and Northerners opposed the potential spread of slavery. However, ultimately, the Missouri Compromise did not prevent continued debate and conflict over the spread of slavery, and by 1854, it was repealed by the Kansas-Nebraska Act, which proved even more controversial and helped spark the Civil War.
https://www.history.com/topics/abolitionist-movement/missouri-compromise