Here’s a quote from the acknowledged Father of the Constitution :
“American citizens are instrumental in carrying on a traffic in enslaved Africans, equally in violation of the laws of humanity and in defiance of those of their own country. The same just and benevolent motives which produced interdiction in force against this criminal conduct will doubtless be felt by Congress in devising further means of suppressing the evil.”
- James Madison, State of the Union, 1810
To be clear, Madison is here talking about the importation of slaves from Africa, not their continued enslavement here or the selling of slaves inside the USA.