In the aftermath of the assassination of President John F. Kennedy, the U.S. Congress seriously took up the issue of Presidential succession and the process by which a sitting President could be removed from office, on the basis of incapacity to serve, due to physical or mental illness.
The U.S. Constitution was vague on the precise issue of succession and incapacity, and on several prior occasions, it led to potential crisis. In 1841, when President William Henry Harrison died in office, his Vice President, John Tyler, assumed the Presidency, even though the Constitution only provided for the Vice President to be "Acting President." When President Woodrow Wilson had a mental breakdown (widely attributed to a stroke) 18 months before his term expired, the First Lady and key White House advisors ran the country, without ever officially acknowledging that the President was incapacitated.
In 1963, Senators Estes Kefauver and Kenneth Keating proposed a Constititutional amendment, to authorize the Congress to legislate Presidential succession and rules for removing a President from office for incapacity. That effort was blocked out of fear that Congress could abuse the power. However, on Jan. 1, 1965, Senator Birch Bayh and Representative Emanuel Cellar introduced a Constitutional amendment, spelling out the procedures for succession and for determination of a President's incapacity. The amendment passed both Houses of Congress on July 6, 1965, and was sent to the states for ratification. On February 10, 1967, the required 38 states ratified the amendment.
The relevant section, dealing with Presidential incapacity, read as follows:
Section 4. Whenever the Vice President and a majority of either the principal officers of the executive departments or of such other body as Congress may by law provide, transmit to the President pro tempore of the Senate and the Speaker of the House of Representatives their written declaration that the President is unable to discharge the powers and duties of his office, the Vice President shall immediately assume the powers and duties of the office as Acting President.
Thereafter, when the President transmits to the President pro tempore of the Senate and the Speaker of the House of Representatives his written declaration that no inability exists, he shall resume the powers and duties of his office unless the Vice President and a majority of either the principal officers of the executive department or of such other body as Congress may by law provide, transmit within four days to the President pro tempore of the Senate and the Speaker of the House of Representatives their written declaration that the President is unable to discharge the powers and duties of his office. Thereupon Congress shall decide the issue, assembling within forty-eight hours for that purpose if not in session. If the Congress, within twenty-one days after receipt of the latter written declaration, or, if Congress is not in session, within twenty-one days after Congress is required to assemble, determines by two-thirds vote of both Houses that the President is unable to discharge the powers and duties of his office, the Vice President shall continue to discharge the same as Acting President; otherwise, the President shall resume the powers and duties of his office.
On two occasions, there was serious consideration of invoking Section 4: On March 30, 1981, when President Ronald Reagan was shot by would-be assassin John Hinckley, there was brief consideration of invoking Section 4, and an ad hoc group of close Presidential advisors, at Reagan's bedside, following his surgery, concluded that he was mentally fit to remain in office. In 1987, when Donald Regan was replaced by Howard Baker as President Reagan's Chief of Staff, Baker was urged to make a determination whether Reagan was still mentally sharp enough to serve out his term. After an initial White House meeting between the President and the new staff, Baker concluded that President Reagan was perfectly capable of continuing as President.