Slavery had been a deeply dividing issue since the birth of the nation

At the Constitutional Convention (1787), it was a major point of contention over state representation in Congress
Slave states argued that their slaves should count toward their population when calculating representation in Congress
Free states argued that slaves were not afforded citizenship rights and therefore should not be allowed to count toward a state’s population size
Eventually, the Convention passed the Three-Fifths Compromise, which declared that slaves would count as 3/5 a person when counted toward a state’s population size
The Convention also allowed states to choose for themselves whether slavery was permitted in their territories

Why did these states not want slaves counted toward state population size?

Free states did not want slaves counted toward state population size for several reasons. Firstly, they believed that slaves were not treated as equal citizens, nor were they allowed to participate in the political process. Therefore, it was unfair to give slave states more political power simply because they owned more slaves. Secondly, free states were concerned that if slaves were counted toward population size, slave states would have a disproportionate amount of power in Congress, which could lead to laws and policies that were detrimental to the interests of free states. Lastly, many abolitionists in free states believed that slavery was a grave moral wrong and wanted to limit its influence in American politics and society as much as possible.