Let me write here a prediction for the future:
In less than 50 years from now, a slavery denial movement will raise in the United States.
This will be a significant amount of people who either deny completely that slavery ever existed in the United States, or at the very least will claim that it is extremely exaggerated and that the real situation was not even nearly as bad (in other words, the "slaves" weren't slaves at all; they were just immigrant paid workers who were completely free and treated like anybody else, and at most had signed contracts with their employers that they had to fulfill.)
They will use all the same old tactics as every other denialist and conspiracy theorist has used.
I cannot predict how popular it will become, but it will most probably a relatively fringe movement, comparable to the holocaust denialists (although there's a real possibility that it will become larger due to Christians having an interest in such a thing.) Nevertheless, it will be a real denialist movement with all the crap that will become with it (websites, books, "documentaries", TV celebrities endorsing it, and so on.)