Swapping candidates would look really, really bad… unless he comes up and says “I had a talk with my doctor, he says I’ve developed a chronic condition that I didn’t know about past year. I can’t do it anymore, but here, vote for my VP, who you all already voted for once anyway, and would take over anyway if this thing kills me”.
He can leave out that the chronic condition is just notgonnawinatall, and his doctor is also his pollster.