I don't think that is true. Making people convicted of crimes perform work as punishment happens all the time. True there aren't too many chain gangs any more, but those that have been struck down have done so because of the treatment of the prisoners while working, not the fact that they had to work. Perhaps you were referring to the 13th Amendment which is pretty clear on the issue:
Section 1. 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.
But I agree with you, that the minute a plan like this were to go into effect there would be a gaggle of lawsuits filed by every civil rights organization in the country. It wouldn't last a day before being struck down.
Having captured illegals build the fence would also create the same problems the germans had when they had forced prisoner labor in munition factories. Funny how so many of those munitions where duds.
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Section 1. 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.
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Thanks for that correction. But you're right in agreeing that it would never survive lawsuits claiming "cruel or unusual punishment". Not cruel, but most assuredly unusual.
It wouldn't matter anyway. The prisons are full. Some other solution will be needed and the President is taking the first step shoring up border control.