I have long believed that the most powerful antislavery clause in the Constitution is the three/fifths clause, which designates slaves as constituting three-fifths of a person for purposes of congressional representation. Why? Because of its indefensibility and irrationality. If the Northerners had won, and slaves were not counted for purposes of representation, the South probably would not have signed the document. If the Southerners had won, and slaves were counted as 5/5, it would have been distasteful to the Northerners, but they would have swallowed it, and the controversy would be over (and Southern power substantially increased). But by ordaining this cockamamie "Federal Ratio," every time a slave state came up for admission to the Union, the issue of slave representation blew up again and ripped off the scab. It kept the fact of the political injustice of slavery alive, even at times when the moral injustice was being ignored.
Surely the framers could not have intended this effect, right? Well, it's possible. As Jan Lewis observes in her essay "The Three-Fifths Clause and the Origins of Sectionalism (in Finkelman and Kennon, Congress and the Emergence of Sectionalism), it was James Wilson of Pennsylvania who proposed the Federal Ratio. And he was one of the most forceful opponents of slavery in the Convention.