At the heart of the Missouri compromise was the assumption that...

At the heart of the Missouri compromise was the assumption that states would respect the Mason Dixon Line, and all new states created after 1820 would hold the balance in Congress.

Explanation:

The Missouri Compromise, adopted in 1820, was a pact between the abolitionist and slave-owning political groups of the United States, through which Missouri was admitted to the Union as a slave state, dividing the state of Massachusetts to create the state of Maine a non slave state, in order to maintain the balance between both types of states in Congress. In addition, the Mason Dixon Line was adopted, by means of which slavery was prohibited north of it, allowing it to the south of said dividing line.