The Missouri Compromise act was an attempt by the U.S. to balance Congressional power between slave states and free states, as slaveholding states felt their representation was in danger of becoming outnumbered.
When Missouri tried to enter the Union in 1818, it threatened the tight balance of the existing 11 slave and 11 free states, which was crucial in keeping representation balanced in the Senate.
The growing abolitionist movement pereceived this tension as a good chance for proposing an amendment that banned slavery within the territory of Missouri and push forward gradual emancipation.
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