I don't think parties should have the power to fill seats either to be honest.
If there's not sufficient time to organize and hold a special election, I think the parties would be good at selecting a caretaker Senator that holds similar views to the other Senator. Perhaps we could let a Senator designate his or her successor if he or she resigns or is impeached?
How about there's a vacancy until an election can be held? I think allowing parties to internally select an "elected" representative is worse than a vacancy.
