The constitution does actually explicitly establish convicts as a specific underclass; in fact it literally specifically allows them and only them to be slaves. The 13th amendment reads:
> Neither slavery nor 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.
> Neither slavery nor 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.