This would be a matter for each state who controls their own electors. They don't have to be elected by popular vote except by the laws of the specific states.

However the aftermath of openly question the authenticity would be devastating to the country - and therefore unlikely to be attempted. The best strategy - given where we are is to build enough doubt to weaken Trump's mandate, yet not enough to throw the entire system into chaos. Though even that is an extremely risky game.