The health situation of Senate Minority Leader Mitch McConnell (R-KY) is a hot issue now (see below). Election statistician Nate Silver, founder of FiveThirtyEight, just wrote: "This election is probably going to be close, and Trump might be only one Biden-has-a-McConnell-moment away from winning." It's probably true. Many people, even Democrats, are worried about Joe Biden's age—and the possibility of Kamala Harris having to sit in the big chair if he can't does not fill many people in either party with a sense of joy.
What would happen if Biden had a major health scare? What if he froze up during a speech for 20 or 30 seconds and had to be helped away from the podium by an aide? The only news story for weeks would be: "Biden is too sick to be president." He could have the White House physician issue all the glowing reports he wanted to, but Biden would be toast.
Biden appears to be in good shape for an 80-year-old, sticks to a healthy diet, and has the best medical care in the entire world. But he can't go out and play a couple of sets of tennis or some touch football à la JFK to demonstrate his health. He can keep his campaign appearances limited to short speeches at friendly indoor venues, like union halls, but there is no practical way to pretend he is 60 instead of 80.
If Biden had a health incident, a lot would depend on when he had it. If it were next week or month, he would have the option of dropping out of the race, creating a free-for-all as 20 Democratic POTUS wannabes jumped in. That might result in a stronger candidate—say, Gov. Gavin Newsom (D-CA) or Gov. Gretchen Whitmer (D-MI). But if Biden has a health incident after the Democratic National Convention, then what? If he formally dropped out, the DNC could pick a new candidate, again possibly Newsom or Whitmer (or a Newsom/Whitmer ticket), but would that be legitimate in the eyes of the voters, since they wouldn't have had a trial by fire in the primaries?
While Biden can have a physical exam every month if he wants to and he can instruct the White House chef to put more spinach and fewer fries on his dinner plate, some things are out of his control and he has to live with them. One thing he could do, though, is run ads mentioning that he is only a couple of years older than Trump and lives a much healthier lifestyle than Trump does. The ads could show Biden well dressed and healthy looking and Trump as a disheveled fat slob.
One thing that Silver didn't mention is that it could go the other way as well: Trump could have a McConnell moment. Then the shoe would be on the other foot. Fate doesn't play politics. (V)