I am not convinced that the election process is the ultimate check on the presidential authority. What if they _become_ insane during the term, join the secret

I am not convinced that the election process is the ultimate check on the presidential authority. What if they _become_ insane during the term, join the secret Korean cult, mindwashed by Russians, or just generally lose their bearings in completely unpredictable way? I only can hope that there is some classified procedure designed to deal with this situation, otherwise, human civilization is doomed, sooner or later.