The Twenty-Fifth Amendment to the Constitution provides the mechanism for Presidential succession by the Vice President in the event the Office of the President becomes vacant or the President becomes unable to perform the duties of his office.
Upon the death, resignation, or removal of the President, the Vice President immediately and automatically assumes the Office of President and relinquishes the office of Vice President. The taking of the oath of office is not a necessary precondition to assuming the office of the President under these circumstances, but is an obligation which should be promptly discharged.
For purposes of declarations that the President is unable to discharge the powers and duties of his office under § 4 of the Twenty-Fifth Amendment, “the principal officers of the executive departments” are the heads of the departments listed in 5 U.S.C. § 101.
The written declarations of Presidential inability triggering succession procedures under § 4 of the Twenty-Fifth Amendment need not necessarily be personally signed by the Vice President and a majority of the principal officers of the executive departments. The only requirements are that their assent to the declaration be established in a reliable fashion and that they direct that their names to be added to the document.