FILE - The U.S. presidential seal is seen on a podium at MacDill Air Force Base, in Tampa, Florida, Dec. 6, 2016.
FILE - The U.S. presidential seal is seen on a podium at MacDill Air Force Base, in Tampa, Florida, Dec. 6, 2016.

The 25th Amendment to the Constitution of the United States details the process by which the president or vice president is replaced in the event either person is not available for service.

Reasons to replace the head of state or the vice president — the highest-ranking official in the presidential line of succession — are death, dismissal from office, resignation or incapacitation. Examples include the November 1963 assassination of President John F. Kennedy and the 1974 resignation of Richard Nixon in the midst of the Watergate scandal.

New calls to invoke the 25th Amendment were sparked by Wednesday’s mob attack on the U.S. Capitol building by supporters of President Donald Trump. Incoming Senate Majority Leader Chuck Schumer was among those calling for Vice President Mike Pence to use the amendment to remove Trump from office.

The amendment's language on what could result in the president's involuntary removal from office is terse. The amendment's fourth (and untested) section simply says the vice president and a majority of the Cabinet (or a congressional body) could act to remove him "when the president is unable to discharge the powers and duties of his office," adding the vice president will immediately assume those powers.

To achieve this, the vice president and the majority of the Cabinet must first submit a written declaration of the commander-in-chief's inability to perform to the president pro tempore of the Senate - the chamber's second-highest ranked member - and the Speaker of the House of Representatives.

If the president subsequently determines he is able to resume his duties, he must declare his fitness for office in writing to the Senate president pro tempore and the House speaker. The president would resume his duties unless the vice president and a majority of the Cabinet inform Congress in writing within four days of the president's letter that the president is still unable to perform his duties. Congress would then convene within 48 hours and have up to 21 days to deliberate.

A congressional determination that the president is not able to execute his duties must be decided by a two-thirds majority in the House and Senate.

If the two-thirds thresholds are not met, the president would be allowed to "resume the powers and duties of his office," the amendment states.