Why might southerners be more upset if California were admitted as a free state than northerns?

Southerners might be more upset if California were admitted as a free state than northerners because it would upset the balance of power in the Senate, where each state is equally represented regardless of population. California was a large and populous territory expected to be admitted as a free state, and this would give the free states a majority in the Senate, potentially shifting the balance of power in favor of the anti-slavery faction in the United States. This would increase tensions between the North and the South and make southerners fear their political interests would be overlooked.