CRS report via LC – Presidential Disability Under the Twenty-Fifth Amendment: Constitutional Provisions and Perspectives for Congress, Updated November 5, 2018:  “Sections 3 and 4 of the Twenty-Fifth Amendment to the U.S. Constitution provide for presidential disability or inability. Section 3 of the amendment sets the procedure whereby a President may declare himsel for herself “unable to discharge the powers and duties” of the office by transmitting a written declaration to this effect to the President pro tempore of the Senate (President pro tem) and the Speaker of the House of Representatives (Speaker). For the duration of the disability, the Vice President discharges the President’s powers and duties as Acting President. When the President transmits “a written declaration to the contrary” to the President pro tem and the Speaker, he or she resumes the powers and duties of the office. Section 3 is intended to cover either unanticipated disability, such as injury or illness, or anticipated disability, such as medical treatment. It has been activated three times under circumstances in which the President underwent general anesthesia for medical treatment. It was informally implemented by President Ronald Reagan in 1985 and was formally implemented twice by President George W. Bush, in 2002 and 2007, under similar circumstances.

Section 4 provides for instances of contingent presidential disability. It was intended by the Twenty-Fifth Amendment’s authors to provide for cases in which a President was unable or unwilling to declare a disability. In these circumstances, the section authorizes the Vice President and a majority of either the Cabinet, or such other body established by law (a presidential disability review body), acting jointly, to declare the President to be disabled. When they transmit a written message to this effect to the President pro tem and the Speaker, the Vice President immediately assumes the powers and duties of the office as Acting President…”