Under Section 4 of the 25th Amendment:
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.
In 1985, the Justice Department’s Office of Legal Counsel explained that:
We believe that the “principal officers of the executive departments” for the
purposes of the Twenty-Fifth Amendment are the heads of the departments
listed in 5 U.S.C. § 101. . . .
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