Answer:
The admission of new states created unevenness within the congress. This means that when a new state was added, if that state was slave or free, it would allow the congress to rush bills to pass for one side.
Prior to the Civil War there were 3 big compromises.
1820, The Missouri Compromise - Created the 36'30' Line, splitting slave and free states and for Missouri to be admitted as a slave state, Maine had to be admitted as a free state.
The compromise of 1850 - Admitted California as a slave state, and all territories gained from the Mexican Cession were to be unassigned as free or slave. And strengthened the Fugitive Slave act, allowing southerners to go into the North to get back escaped slaves.
And Finally the Kansas-Nebraska Act of 1854 - Allowed Kansas and Nebraska to choose whether they're free or slave by popular sovereignty. This lead to "Bleeding Kansas" A point in which both pro and anti-slavery people flocked to Kansas to have it admitted as the state of their choice, using violence to silence the other side.
