...and one of the candidates didn't show up? It's probably happened before, but never in a Presidential contest. It's certainly possible that a Presidential nominee could die, or otherwise be incapacitated, or resign due to a late-breaking scandal, just before the election. It would probably turn out differently depending whether it happened before or after the party convention, but either way it would mean any last minute nominee would inevitably have to be picked in the traditional smoke-filled room. Perhaps the vice presidential candidate could step into the breach, but what if one hadn't been picked yet?

Just a thought...