In 1819 the U.S. had an equal number of slave and free states (eleven each). States were generally divided along the Mason – Dixon line. The Northern States (free) had greater population, and thus greater representation in the House of Representatives, but the Senate was equally divided. The population of the North was growing faster than the population in the South, so if the two sections were to remain equally balanced, it could only occur in the Senate, where each state was equally represented regardless of population. No move had been made to extend slavery into territory gained by the Louisiana Purchase. That year, Missouri wanted to join the Union as a state. Representative James Talmadge of New York introduced a resolution that prohibited further introduction of slaves into Missouri, and freedom at age 25 for those born into slavery after its admission as a state. (at the time, Missouri already had 10,000 slaves.) The resolution passed the House, but failed in the Senate, largely along sectional lines.
Maine had applied for admission also, so compromise was worked out:
- Slavery was excluded from balance of Louisiana Purchase north of 36° 30’N. (Southern Border of Missouri.) This appeared to be a victory for the South, as the area of the American West had been designated the Great American Desert, unfit for agriculture, or for anything but Indians and rattlesnakes, not necessarily in that order.
- Maine would join Union as free state, Missouri as slave state this meant that the slave state/ free state division would remain along sectional lines.
Another problem arose when proslavery interests in the Missouri Constitutional Convention insisted the State Constitution provide that free blacks and mulattoes would be excluded from the state. (This was a clear violation of the Federal Constitutional provision that protected "privileges and immunities" of U.S. Citizens.) Free states were citizens of several other states. This was an additional controversy that almost blocked Missouri from joining the Union.
Henry Clay developed a "Second Missouri Compromise:" The offending clause could remain but the legislature must agree that it would never be construed so as to deny privileges that citizens held under the U.S. constitution. (In other words, the Constitution did not mean what it said.) Missouri was subsequently admitted to the Union as the 24th State in 1821.