I believe there’s an act covering presidential disability, dating from long before Reagan, due to a president’s wife having effectively run the country for a couple of years while her husband was too ill to get out of bed. That would probably cover obvious and serious dementia as well. (Not my country, though, so I may have it wrong.) Problem with the recent Republican presidents is that their insanity is plausibly deniable, if your worldview is damaged enough already.