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California's admission to the Union was bitterly contested because:
California was told it had to come in as a slave state, despite being almost entirely slave-free.
Southerners didn't want to admit California, as it would come in as a free state, which would give slave-free states control of both houses of Congress.
California's admission would spark another war with Mexico, which was still bitter over losing the last war.
California's admission would spark a war with Spain, which still saw the territory as theirs.
Southerners believed the territory was actually part of Texas, and therefore could not be split off as its own state.