If you’re not very familiar with the 25th Amendment to the United States Constitution, you might want to review this bit from Section 4 — it’ll become very relevant during this week’s Madam Secretary:
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.
And if you didn’t quite follow, don’t worry. Sunday’s episode (10/9c) of the CBS drama will make everything quite clear: The 25th Amendment provides the framework for replacing an incapacitated American president, a scenario with which Elizabeth and Russell potentially face after Dalton starts acting wildly out of character.