US History: Chapter 5, Exercise 1 — Possible Solution
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WHY NECESSARY
By 1819 the Senate was evenly split, 11 slave states to 11 free, with two
senators each regardless of population. Admitting Missouri as a slave state
alone would have tipped that balance, so it was paired with Maine's admission
as a free state, plus the 36 30 line for future territories.

WHAT CHANGED IN 1854
The Kansas-Nebraska Act replaced the fixed geographic line with "popular
sovereignty," letting settlers in each territory decide slavery for
themselves, effectively repealing the Compromise's restriction.