Why was it necessary to maintain a balance of power between the Slave and Free states?

Question

As said in the title, why was there a need to maintain the balance of power between the Slave states and Free States by keeping them equal in number, before the American Civil War? Both the Clay Compromise and the Missouri Comprise were aimed at achieving this.

Answer 1

The short answer is that in order to keep the Southern states happy, the North had to agree to bring in a new slave state for every free state into the Union. The Missouri compromise of 1820, for example, brought in the new state of Main (where slavery was outlawed) as well as Missouri (a slave state).

The balance of power was in the Senate. That balance held until the Civil War, when the South was no longer convinced that the North would allow southerners to expand their slave economy to new territories.

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