U.S. Const. amend. XX, § 4

Congress to provide for case wherein death occurs among those from whom House chooses a President.

The Congress may by law provide for the case of the death of any of the persons from whom the House of Representatives may choose a President whenever the right of choice shall have devolved upon them, and for the case of the death of any of the persons from whom the Senate may choose a Vice President whenever the right of choice shall have devolved upon them.