If they want to, they can. They won't because he'll be braindead by this time next year, much less beyond, and it won't be worth it.
But I should have said that if they say he's fine and just has a stutter, then his handling of classified documents is just as criminal, if not more so, than Trump's. And that's gonna impact his re-election chances.
As for being the 14th best POTUS, they listed Carter 22nd. That tells you all you need to know.