From: History Channel 

The Missouri Compromise was an effort by Congress to defuse the sectional and political rivalries triggered by the request of Missouri late in 1819 for admission as a state in which slavery would be permitted. At the time, theUnited States contained twenty-two states, evenly divided between slave and free. Admission of Missouri as a slave state would upset that balance; it would also set a precedent for congressional acquiescence in the expansion of slavery. Earlier in 1819, when Missouri was being organized as a territory, Representative James Tallmadge of New York had proposed an amendment that would ultimately have ended slavery there; this effort was defeated, as was a similar effort by Representative John Taylor of New York regarding Arkansas Territory.

The extraordinarily bitter debate over Missouri's application for admission ran from December 1819 to March 1820. Northerners, led by Senator Rufus King of New York, argued that Congress had the power to prohibit slavery in a new state. Southerners like Senator William Pinkney of Maryland held that new states had the same freedom of action as the original thirteen and were thus free to choose slavery if they wished. After the Senate and the House passed different bills and deadlock threatened, a compromise bill was worked out with the following provisions: (1) Missouri was admitted as a slave state and Maine (formerly part of Massachusetts) as free, and (2) except for Missouri, slavery was to be excluded from the Louisiana Purchaselands north of latitude 36°30'.

The Missouri Compromise was criticized by many southerners because it established the principle that Congress could make laws regarding slavery; northerners, on the other hand, condemned it for acquiescing in the expansion of slavery (though only south of the compromise line). Nevertheless, the act helped hold the Union together for more than thirty years. It was repealed by the Kansas-Nebraska Act of 1854, which established popular sovereignty (local choice) regarding slavery in Kansas and Nebraska, though both were north of the compromise line. Three years later, the Supreme Court in the Dred Scott case declared the Missouri Compromise unconstitutional, on the ground that Congress was prohibited by the Fifth Amendment from depriving individuals of private property without due process of law.