election fraud and voter fraud are criminal offences.
contested elections are a different matter...and different standard of proof
You specifically stated: what is the standard proof in election fraud cases..criminal case?
Contested elections are based on the legality or validity of the results but either requires proof one way or the other. We can argue all day but the lawyers i.e. Sidney Powell, Lin Wood etc law suits specifically state “massive election fraud” and “wide-spread election fraud” and in Trumps own words he states “election fraud” and election fraud is a criminal offense and that’s what you asked standard of proof wise.
Contested elections on validity are “usually” (NOT ALWAYS) a small voter number differences like the one for NY 22nd district and Republican Claudia Tenney finally wining by 109 votes give or take.