Besides the process of impeachment — which triggers a trial in which a president defends himself before a jury of senators — there is another, less-mentioned method by which a president can be removed from office. That method is provided the 25th Amendment of the U.S. Constitution. The 25th Amendment states the conditions under which Congress, in cooperation with the vice president and other administration members, would be empowered to remove a president from power.
Section four of the amendment states,
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.
Now, first thing's first. If you're wondering what "President pro tempore of the Senate" means, it's essentially the temporary president of the Senate. The actual president of the Senate is the vice president, but the vice president isn't always physically at the Senate, in which case a different senator takes charge and become president pro tempore.
Secondly, if you're wondering what the Cliff Notes version of section four is, don't worry, it's much simpler: A vice president and a president's cabinet can relay to both chambers of Congress — the House and Senate — that a president is unable and unfit to perform the duties that the office demands.
The section doesn't stop there, though. It outlines provisions for a situation in which the president argues against the judgement of his vice president and cabinet. According to the 25th Amendment, if the president conveys to Congress that the he is — contrary to the assertion of his administration — able to carry out his duties, then the president will reassume power.
Section four states,
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.
At that point, however, a president's administration can re-assert its claim that the president is unfit for office. If and when this claim is brought before Congress again, Congress then has to resolve the matter itself.
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.
Translation? Congress can decide via a two-thirds vote whether a president should be removed from office, in which case the vice president would assume the role of commander in chief.
There have been critics of current President Donald Trump who have argued that the 25th Amendment should be invoked for this presidency.
One of those critics is conservative New York Times columnist Ross Douthat, who argued Trump's "omni-incomptence" provides grounds for Congress and Trump's cabinet to declare him unfit.
Any scenario in which section four of the 25th Amendment is invoked to remove a president from power would represent a first in U.S. history.
Former President Richard Nixon invoked the 25th Amendment to appoint his vice president, but that decision was in accordance with section two of the amendment, which states the president has the power to appoint a vacancy in the vice presidency, subject to the approval of Congress.
As for section four? It has never been used to remove a POTUS, partly because it was created out of fear that a president would become unable to perform duty because of health issues, not as a means to terminate the tenure of an unpopular president. Still, the section and the amendment's existence makes it possible to remove a president without the process of impeachment.