Death or failure to qualify of the President elect
Official text rightpowerlimitif conditionlegal term
If, at the time fixed for the beginning of the term of the President, the President elect shall have died, the Vice President elect shall become President. If a President shall not have been chosen before the time fixed for the beginning of his term, or if the President elect shall have failed to qualify, then the Vice President elect shall act as President until a President shall have qualified; and the Congress may by law provide for the case wherein neither a President elect nor a Vice President elect shall have qualified, declaring who shall then act as President, or the manner in which one who is to act shall be selected, and such person shall act accordingly until a President or Vice President shall have qualified.
Proposal and ratification: The twentieth amendment to the Constitution was proposed to the legislatures of the several states by the Seventy-Second Congress, on the 2d day of March, 1932, and was declared, in a proclamation by the Secretary of State, dated on the 6th day of February, 1933, to have been ratified by the legislatures of 36 of the 48 States. was completed on January 23, 1933.
Simplified explanation
If the President elect has died, the Vice President elect becomes President. If no President has been chosen or the President elect fails to qualify, the Vice President elect acts as President until one qualifies; Congress may by law provide for the case where neither qualifies.