[Reily]

Severe penalties for those who directly commit vote fraud!
Leave each state to decide what severe means.
Also I would recommend the following for candidates whose supporters are caught doing vote fraud. Immediate forfeiture of office if the candidate is the winner, with a re-election, and the offending candidate must sit out one election cycle. If such penalties existed candidates would make darn sure that their staffs functioned on the up & up!

[Walkingfeather]

Well there would have to be a way that the opponent does not frame a candidate in voter fraud in order to take them out.