Certify the election results (presumably due to clear and convincing evidence of either fraud or that the voting or counting process broke down and cant be relied upon). That would justify the legislature tossing aside the election results and appointing the state’s electors themselves.

This would have to be done in at least three states and would presumably require GOP control of both state legislative chambers and the Governor (as I’d think any legislation to actually do this could be vetoed). Oh, and the courts would have to go along as well.

It’s an extremely remote scenario and, IMHO, would constitute the most overt attempt at overthrowing the most sacred democratic process in the history of the nation. It almost makes me want to see them try it as I think the future ballot box blowback could cripple them for a generation even if the attempt failed.