The US Constitution’s Article 2 says the vice President assumes the “powers and duties” of the President in the event of the President’s “inability,” but it doesn’t say how it’s determined that a President is unable to serve.
Article 2 section 1 (6) states, in case of the removal of the President from office, or of his death, resignation, or inability to discharge the powers and Duties of the said office, the same shall devolve on the vice President, and the congress may by Law provide for the case of removal, death, resignation or inability, both of the President and vice President, declaring what officer shall then act as President, and such officer shall act accordingly, until the disability be removed, or a President shall be elected.
Therefore the 25th Amendment was ratified in 1967 in the wake of President John F. Kennedy’s assassination in 1963. The intention of this amendment was to deal with any ambiguity about who leads the government when a President cannot. Thus it requires the President, if he determines that he is unable to discharge the duties of his office, to inform Congress, in writing, of his intention to voluntarily relieve himself of his duties. The vice President in that event would assume the duties of the Presidency, as the “acting President,” until such time that the President informs Congress, again in writing, that he can resume his duties.
Vice President assumes “powers and duties” of the President in the event of the President’s “inability,” but it doesn’t say how it’s determined that a President is unable to serve
Section 3: Whenever the President transmits to the President pro tempore of the Senate and the Speaker of the House of Representatives his written declaration that he is unable to discharge the powers and duties of his office, and until he transmits to them a written declaration to the contrary, such powers and duties shall be discharged by the Vice President as acting President.
This is where he voluntarily transfers power. However section 4 spells out how in a situation where he ought to but does not transfer powers which is known as involuntary transfer of power. Under this section it allows the President to be challenged from resuming power if the vice President and the President’s Cabinet thought he was incapable of governing. In that unusual scenario, if the vice President and a majority of the “Principal officers of the Executive Departments” can make a determination that the President is unable to carry out the duties of his office, and they can send a written declaration to Congress to that effect and install the vice President as the acting President.
However, the President is entitled to contest this decision. Hence if the President contests his removal, the vice President and a majority of the Cabinet must again declare in writing the President incapable of his duties within four days. Congress must then convene within 48 hours and has 21 days to decide the outcome. It would require a two-thirds majority vote of both chambers of Congress to declare the president unable to carry out his duties and should remove him from office. If the vote fails, the President remains in office with all the powers and duties that entails.
In the US, there are situations where section 3 has been tested but not section 4. Many other Presidents have fallen seriously ill while in office or died of natural causes while in the White House, but the process of transferring power from a living President to vice President has come up only three times in history. Each time, the President gave up his duties voluntarily before planned surgeries that included sedation or anesthesia.
Ronald Reagan transferred power to Vice President George H.W. Bush in 1985 for eight hours while he underwent surgery for colon cancer
President Ronald Reagan transferred power to Vice President George H.W. Bush in 1985 for about eight hours while he underwent surgery for colon cancer. Twice, in 2002 and 2007, President George W. Bush invoked Section 3 prior to going under anesthesia for routine colonoscopies, transferring presidential duties to Vice President Dick Cheney for roughly two hours each time. President Bush specifically cited the 25th Amendment when declaring his disability and reclaiming his authority in his declaration to Congress.
In 1947, President Harry Truman signed the Presidential Succession Act. The original act of 1792 had placed the Senate President pro tempore and Speaker of the House in the line of succession, but in 1886 Congress had removed them. The 1947 law reinserted those officials, but placed the Speaker ahead of the President pro tempore.
Therefore, in the current situation if both President Trump and vice President Mike Pence were unable to perform the duties of President, there would be a written declaration, and House Speaker Nancy Pelosi would be required to resign both as speaker and as a representative in Congress in order to become acting President. President pro tempore of the Senate Sen. Chuck Grassley, (R-Iowa) would be next in line after Pelosi.