What happens if a presidential nominee can't perform his role prior to election who would replace him/her?

Question

We know the order of succession after election but what happens between now and then if someone becomes incapacitated.

Answer 1

The Vice President canidate would take over.

Answer 2

All the would-bes would draw straws.

Answer 3

lol thinking aout what is going to happen huh

Answer 4

If Obama is out Hillary is back

if McCain is out Mitt Romney is back