If the legality of the election is under question, why would it be unconstitutional? Say it's proven that voting machines were rigged in Wisconsin plus 3 other close States? Would you accept that it still was a 'valid' Democratic choice made by American people? It obviously is not. Why is the Constitution expected to uphold a election that was rigged by Russians? Doesn't make sense.
All you can say is that the Constitution doesn't cover this scenario, but that in itself doesn't make it unconstitutional.
Bear in mind I think its batshit mental to stick by a 200+ year set of rules regardless of common sense, but just deciding they're going to overturn the constitutional mechanisms for elections and decide to hold a new one would certainly be unconstitutional. It's basically just making up a new rule, and under that system you can't just do that.