‘make any Thing but gold and silver Coin a Tender in Payment of Debts (emphasis added); ‘
which would make our paper money illegal. Unless the argument is made that paper money represents or is backed by physical gold and silver, which it used to be obviously. But now, there is no defense for paper money or credit even. hmmm...
Yes, unless the states amended the Constitution to allow for paper money. But this is probably a bad idea as evidenced by hindsight.