One of the most critical and intriguing constitutional questions ever presented for solution is: What happens when the President of the United States becomes incapable of discharging the powers and duties of his office? Does the Vice-President "become President" for the remainder of the term or does he merely "act as President" during the period of the inability? The Constitution is not explicit..
John D. Feerick,
The Problem of Presidential Inability—Will Congress Ever Solve It?,
83 Fordham L. Rev. 2891
Available at: https://ir.lawnet.fordham.edu/flr/vol83/iss6/2