Final answer:
Gerald Ford is the only president in US history who was never elected to the office of either president or vice president, ascending to the presidency through the provisions of the Twenty-Fifth Amendment after Nixon's resignation.
Step-by-step explanation:
The only president to never be elected by the voting public to either the president or vice president was Gerald Ford. He became vice president after Spiro Agnew resigned following a corruption scandal, and later assumed the presidency upon the resignation of President Richard Nixon during the Watergate scandal.
This was in accordance with the Twenty-Fifth Amendment, which allows for the appointment of a new vice president in the event of a vacancy. Ford is notable for being the first to ascend to the presidency in this manner, and for being the only individual to have served as president without having been elected to either the presidency or the vice presidency.