The 13th amendment states: "involuntary servitude, except as a punishment for crime whereof the party shall have been duly convicted, shall exist within the United States, or any place subject to their jurisdiction."

We have an immense prison population.

We have lots of explicit slaves 

 Agreed. And then the "free" ones are obligated to work almost half the year for someone else's benefit. Resist and get thrown in a cage.