> What was new was the movement to ban slavery, especially in new states
Abolition wasn't a new thing in 1861. All the northern states had banned it in the decades after the revolutionary war. Indeed, slavery was banned in Georgia in 1735 when it was founded. The British Empire abolished slavery in 1833. Southern states in 1861 were fighting to preserve an institution that many people had recognized as evil for more than a century.
> which would undo the political balance that had been put in the Constitution from the beginning. (The Constitution still gives extra power to those states.)
You're mixing up two different things. The creation of a House with representation based on population and a Senate with a fixed number of Senators per state, which continues to exist today, was a compromise between big states and small states. Four of the seven states that were smaller than average and benefited from that arrangement had few to no enslaved people: New Hampshire, Rhode Island, and New Jersey and Connecticut. Conversely, three of the six states that were bigger than average and were hurt by that arrangement enslaved large numbers of people.
Additionally, the Constitution did not "give extra power" to slave states. It reduced the power of the slave states using an anti-slavery argument. Look at the text of the 14th amendment, which governs apportionment:
> Representatives shall be apportioned among the several States according to their respective numbers, counting the whole number of persons in each State, excluding Indians not taxed.
Compare that to the 3/5 clause:
> Representatives and direct taxes shall be apportioned among the several states which may be included within this union, according to their respective numbers, which shall be determined by adding to the whole number of free persons, including those bound to service for a term of years, and excluding Indians not taxed, three fifths of all other Persons.
Both then and now, the Constitution allocates votes based on the number of "persons" in each state, not just people who can vote. So today, the apportionment is based on a population count that includes children and non-citizens, not just adults over 18. Moreover, even in 1789, the Constitution deemed enslaved people to be "persons." It distinguished between "free persons" and "all other Persons" but both were "persons." So what would have happened if the 14th amendment text was in place in 1789? Slave states would have had even more representation, because the "number of persons in each State" would include enslaved people.
The anti-slavery argument was this: because slaveholding states treated some people like property, enslaved people should not be included in the "number of persons in each State." It partially succeeded in reducing the power of slave-holding states. It was a compromise with the devil, no doubt. But the idea that the Constitution increased the power of the slave states is false. Without the 3/5 clause, the slave states would have had even more power.
