Wikipedia

Among potential examples of such unforeseen emergencies, legal scholars have listed kidnapping of the president and "political emergencies" such as impeachment. Traits such as unpopularity, incompetence, impeachable conduct, poor judgment, or laziness might not in themselves constitute inability, but should such traits "rise to a level where they prevented the President from carrying out his or her constitutional duties, they still might constitute an inability, even in the absence of a formal medical diagnosis." In addition, a president who already manifested disabling traits at the time he was elected is not thereby immunized from a declaration of inability.