Why do California's request to become a state cause a crisist in congress?

In California's state constitution they wanted to be admitted into the union as a free state because miner during the gold rush didn't want to have to compete with slave labor. Southern slave owners didn't want that because it would offset the 15 free states and 15 slaves state balance in the senate. If another free state was added the south would lose power. The south feared if they lost power northerners would abolish slavery.