Not really no, a lot of people regarded slavery as morally reprehensible in the 18th century, so to claim they didn't know what we know is a false position.
The educated ones in Britain and its colonies (judging by your use of Founding Fathers, presidents and mentioning revolutionary era, I presume you are a Yank) would have been aware of the moral opposition to slavery and can't be said to just be on the opposite side of a political fence.
In Britain the Abolitionist movement was led by Wilberforce (who would be left wing by today's standards) and taken up by Pitt the Younger (who would be right wing). It was a moral choice, often coloured by economic realities, but to claim it was a political position is probably jumping the gun, that seems to have come later in the US.