California was the biggest of all new states and likely candidates to enter the Union, though it was not very populated by then. However, its population was rising fast because of the golden rush and significant immigration from the East Coast after 1848.
The Southern states, which allowed and supported slavery, were worried that the incorporation of California as a free state would alter the delicate balance of power between free states and slavery states in Congress and in general. Let´s not forget that slavery was very important to the South because its economy was agricultural and demanded a large labor force to function.
