After Richard Nixon resigned in the wake of the Watergate scandal, Gerald Ford, who had been a Congressman, was elevated to the office of the presidency, despite having not been elected to either office.
"Some people say" that Ford's massive gaffe during the Presidential Debate against Jimmy Carter doomed his reelection. There are others that say it was his own clumsiness, or perception thereof, that doomed his reelection. Then there's a whole passel of people who blame Chevy Chase's portrayals of Ford on Saturday Night Live as the reason Ford wasn't reelected. I've never listened to those "nattering nabobs of negativity" because I've always believed Ford lost reelection because he pardoned Richard Nixon.
This turn of events made Gerald R. Ford the only appointed President of the United States, until George W. Bush in 2000.