What ended the balance of free of slave states in the union? why did most northerners oppose the fugitive slave act? who benefited from returning slaves to slave holders under the fugitive slave act?

Northerners, most of whom favored "free states" in which slavery was prohibited, feared slavery wouldbecome de facto in the states carved from the Northwest ... be placed on the territories of Utah or New Mexico and passage of the Fugitive Slave Act, which required citizens of all states to return any runaway slaves 

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