In a world where we can conquer a country without a Congressional declaration of war, as required by the Constitution, I would say the Constitutionality of any process is not the primary concern of the politicians.This would probably be impossible though, maybe even un-Constitutional for all I know (states rights?), but otherwise, it's up to the states to each tackle this and try to fix it I believe.
However, a legitimate case can be made that removing "state mandates" is Constitutional under the commerce clause, which was intended to prevent states from engage in trade wars or embargoes with each other; the current system is a massive web of embargoes in effect.