Section 4 of the 25th Amendment
Text of 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 departments 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 twenty-one days after receipt of the latter 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."
To date the provisions of Section 4 of the 25th Amendment have never been implemented. And based on its potential for either mischief, mass confusion, or both if it ever were? That's probably for the best.
Author's Notes and Comments
Okay... where to begin.
Section 4 of the 25th Amendment is by far its most wordy, most complex, and most potentially confusing provision. Written in the middle of the Cold War era, it was drafted with flexibility to address a number of scenarios in mind - from a conventional disability situation to unthinkable scenarios such as a full scale, decapitation style nuclear strike by an enemy of the United States.
But thanks to the way it is worded, depending on the circumstances in which it may someday be implemented, an invocation of Section 4 could be a constitutionally sanctioned coup d'etat, or worse - incapable of being implemented at all. For example, here are a few scenarios which illustrate the potential mischief and/or chaos that could surround a potential Section 4 invocation:
(1) At a joint appearance in the midwest and despite Secret Service protection, President Smith and Vice President Jones are shot at by an assassin. Jones is killed instantly, while Smith is severely wounded and unconscious, his recovery uncertain, perhaps even doubtful. Being unconscious, the President is unable to declare himself incapacitated under Section 3. And thanks to the wording of Section 4, it cannot be invoked either.
As there is no longer a Vice President to join the Cabinet in declaring the President disabled - the provisions of the amendment specifying that "the Vice President and a majority..." of Cabinet officers must do so. In this situation, the Cabinet cannot act unilaterally and a Speaker of the House cannot lawfully act as President. While by law Congress is empowered under Section 4 to designate a substitute "other body" for the Cabinet (envisioned specifically in the event of a decapitation strike), it cannot legally remove a Vice President from this process. So in this scenario, there's no constitutional means for anyone to act as President.
(2) Woeful economic conditions and high unemployment have made President Garcia quite unpopular. With 21 months yet to serve in her term, polls have Garcia's popularity at a scant 19%, and 46% of respondents in the same poll believe Vice President Parker would do better. Sensing an opportunity, Parker meets privately with members of Garcia's Cabinet, convincing them that it's time for Garcia to go. Eight of them join him in invoking Section 4, and with just nine signatures on a sheet of paper, the President of the United States has been at least temporarily removed from power. To add another layer of intrigue, President Garcia responds by immediately declaring that there's no disabling condition under Section 4. Despite this declaration, under Section 4 she's not immediately restored to office - instead, it triggers a four day period (four days having been chosen as a compromise in Congress when the amendment was being debated) during which Vice President Parker and the Cabinet could reiterate that Garcia was disabled.
In this situation, it's implied that Vice President Parker continues to act as President during these four days, but it isn't unequivocally stated. Further, in such a scenario the Congress gets involved, essentially dropping everything in order to address the issue within 21 days. And if Congress sides with Parker and determines Garcia is in fact disabled? Parker acts as President but Garcia can immediately start the whole process over again. And again. And again.
While these scenarios wouldn't be considered typical by any means, they nonetheless remain plausible scenarios in which an invocation of Section 4 of the 25th Amendment could cause government paralysis in the executive branch, lawsuits galore, mass confusion within both the government and the people, and an unprecedented constitutional crisis... all presumably at the worst possible time to have them.