For a better picture about just who defended what and why refer to the Lincoln/Douglas debates.

https://en.wikipedia.org/wiki/Lincoln%E2%80%93Douglas_debates

The 3/5ths clause was a compromise rule because of Slavery. Counting the Slaves would've given the South unfettered rule of the Union forever. https://www.heritage.org/constitution/#!/articles/1/essays/6/three-fifths-clause
Obama loved to harp about it never bothering to tell the truth about just why it was there in the first place. Slaves were not taxed nor had the right to vote. Why then would they be counted in appointing delegates in the first place? At present the Democrats are flooding our Nation with illegals with the same intent they had during Slavery with one caveat; they will be allowed to vote.

Slavery likely would've died had it not been pushed so hard by the Southern Oligarchs; Big Plantation owners that largely funded the Civil War without actually fighting in it. They insisted on spreading it to the new territories which was opposed by Republicans. Most people on both sides never owned a slave and could've ever afforded to purchase one. Most people that fought did so because they were told it was a territorial dispute and they were defending their own territory. Lee fought for the South because he was from Virginia.
I hold no animosity toward anyone on the other side of the debate but defending the idea of slavery is untenable. It is against our own Declaration of Independence, and I agree concessions were made in order to get southern slave states to sign but it was debated and always considered that in time they would abandon the idea of slavery.
holding on to the idea of legalized slavery is just wrong and the arguments for it and against the war are not unlike those poor tortured soles that destroy our Confederate Monuments and Monuments of our Founding Fathers.