<P> Under Section 2, whenever there is a vacancy in the office of Vice President, the President nominates a successor who becomes Vice President if confirmed by a majority vote of both Houses of the Congress . </P> <P> Section 3 provides that when the President transmits a written declaration to the President pro tempore of the Senate and the Speaker of the House of Representatives, stating that he is unable to discharge the powers and duties of the Presidency, and until the President sends another written declaration to the aforementioned officers declaring himself able to resume discharging those powers and duties, the Vice President discharges those powers and duties as Acting President . </P> <P> Section 4 is the only part of the amendment that has never been invoked . It allows the Vice President, together with a "majority of either the principal officers of the executive departments or of such other body as Congress may by law provide", to declare the President "unable to discharge the powers and duties of his office" by submitting a written declaration to the President pro tempore of the Senate and the Speaker of the House of Representatives . As with Section 3, the Vice President would become Acting President . </P> <P> Section 4 is meant to be invoked should the President's incapacitation prevent him from discharging his duties, but he is unable or unwilling to provide the written declaration called for by Section 3 . The President may resume exercising the Presidential duties by sending a written declaration to the President pro tempore and the Speaker of the House . </P>

Who determines if the president is unfit to serve