The Senate shall chuse their other Officers, and also a President pro tempore, in the Absence of the Vice President, or when he shall exercise the Office of President of the United States.

The U.S. Constitution's 25th Amendment addresses what happens to the executive office in any case of removal of the President ...

The Vice President of the United States shall be President of the ... does the Vice President sit in modern times. The President Pro Tempore of the Senate is the formal substitute, but normally ...