Compared with the age of the U.S. Constitution, the 25th amendment is relatively new, having only been passed by Congress on July 6, 1965 and ratified on Feb. 10, 1967. Congress enacted the amendment to clarify what happens when the offices of either the vice president or the president become vacant as described in Article II, Section 1, Clause 6 of the Constitution. 

That clause states that in the "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." However, the clause doesn't provide a definition for what it means for a president to have the "inability to discharge the powers and duties" of the office; nor does it outline the procedure or timeline for the transfer of duties from the president to the vice president.

The lack of specifics wasn't much of an issue, though, until the assassination of President John F. Kennedy on Nov. 22, 1963, according to law professors Brian C. Kalt and David Pozen, who authored an interpretation of the 25th amendment for the National Constitution Center. Kennedy's assassination left a sudden and unexpected vacancy of the presidency and forced Congress to evaluate the vagaries of Article II, Section 1, Clause 6. Led by Indiana Sen. Birch Bayh, Congress composed the 25th amendment, which was then ratified by the states in 1965. 

It didn't take long for the new amendment to be put into practice, according to a summary published by Cornell Law School. After Vice President Spiro Agnew resigned on Oct. 10, 1973, President Richard Nixon nominated Gerald R. Ford to succeed him, in accordance with procedures of Section 2 of the amendment. Then, when President Nixon resigned on Aug. 9, 1974, Vice President Ford immediately took the oath of office for the president and again followed Section 2 procedures to nominate Nelson A. Rockefeller to become vice president. 

What the 25th amendment states

There are four sections of the 25th amendment, as follows: 

Section 1: 

In case of the removal of the President from office or of his death or resignation, the Vice President shall become President.

Section 2: 

Whenever there is a vacancy in the office of the Vice President, the President shall nominate a Vice President who shall take office upon confirmation by a majority vote of both Houses of Congress.

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.

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 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. 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.

How does this apply to the Trump presidency? 

Those calling for the 25th amendment to be invoked for President Trump are looking at Section 4, which clarifies actions to take when a president is unable to fulfill the job duties or refuses to step down from the position. In this case, the vice president and the president's cabinet members or "some other body" of top leaders may declare the sitting president as unfit for office and immediately transfer the presidency to the vice president. 

If the president opposes this action, the deciding group has four days to decide if they agree or want to continue to argue that the president is unfit to serve. If the group decides the president should be removed, Congress must vote and make the final decision. The vice president continues as acting president only if two-thirds majorities of both chambers agree that the president is unfit for the position. 

But while Section 4 of 25th amendment clearly outlines the procedure for removing an unfit, uncooperative president from office, the definition of president's "inability" to lead isn't defined. Therefore, without clear criteria for determining the characteristics of an unfit president, it could prove difficult to remove a president from office. 

Additional resources: 

• You can read the 25th amendment in its entirety here.
• Read more about how the amendment works and how it could be applied to the Trump presidency, from the National Constitution Center.