The whole issue that drove the Missouri Compromise was the admission of Maine to the Union as a 'free' state. In the deal that Henry Clay cobbled together in Feb-Mar 1820, Maine would be admitted as a 'free' state and Missouri as a 'slave' state thus maintaining a balance. At about the same time (May 1820) Congress made foreign slave trading an act of piracy.
The Missouri compromise.
The Missouri Compromise dealt with the expansion of slavery into the western territory (previously known as the Louisiana Territory).
No
Maine became a state where slavery was forbidden at the same time that Missouri became a slave state.
James Tallmadge offered an amendment that forbade further introduction of slaves into Missouri in 1819. The US Senate refused to concur with the amendment. The House passed a bill allowing Missouri into the union as a slave state in 1820. In the same year, the Missouri Compromise passed in the US Congress.
South Carolina was voting to secede
The issue was slavery -- should slavery be allowed in the new state of Missouri? The compromise was to admit Missouri as a slave state and to admit Maine as a free state at the same time, thereby keeping a balance of slave and free states. IMPROVEMENT. Furthermore the Compromise stated that for the future, no other slave state had to be established north of a line represented by the parallel 36° 30', coinciding with the southern boundary of Missouri.
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Missouri was admitted as a slave state at (about) the same time Maine was admitted as a free state. So Maine is probably the answer you're looking for.
The Missouri Compromise agreed to admit Missouri as a slave-holding state, and at the same time agreed to admit Maine as a non-slave-holding state. It also entailed that no states above the latitudinal line 36°30' would be admitted as a slave-holding state, promoting a Congressional balance of power.
There were two Missouri compromises. The one in 1820 determined the slave/free status of new states within the territory acquired from France in the Louisiana Purchase. The second one, the Compromise of 1850 did the same thing for the territories acquired from Mexico after the Mexican war.
South Carolina was voting to secede at the same time that Crittenden was pushing a compromise to settle the slavery question.