25th Amendment: Acting President Is Not President

 What the text of Section 4 makes clear is than under that provision, she can be Acting President only.

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.

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.

A vice-president can only become President under Section 1, which states: In the case of the removal of the President from office, or of his death or resignation, Vice-President shall become President.

An Acting President exercises the "Powers and duties" of the presidency, but does not hold the office of the presidency.

The distinction would hardly be academic, if a vice-president became Acting President with years left in the president's term, as there would be a protracted vacancy in the office of the vice-presidency.

Since ratification of the 25th Amendment in 1967, there have been two brief vice-presidential vacancies: two months in 1973 while Gerald Ford was waiting for Congress to confirm his appointment by President Nixon; and four months in 1974 while Nelson Rockefeller was waiting for Congress to confirm his appointment by President Ford.

This could have become a huge succession problem during the current administration, were the president to have had a stroke and lapsed into a coma, unless he had prepared a memorandum to the vice-president akin to that authored by President Eisenhower, on March 3, 1958, which subsequently was adopted by presidents Kennedy and Johnson.

In the event of inability the President would - if possible - so inform the Vice President, and the Vice President would serve as Acting President, exercising the powers and duties of the Office until the inability had ended.

In the event of an inability which would prevent the President from so communicating with the Vice President, the Vice President, after such consultation as seems to him appropriate under the circumstances, would decide upon the devolution of the powers and duties of the Office and would serve as Acting President until the inability had ended.

https://spectator.org/25th-amendment-acting-president-is-not-president/