Well, in the 20th century, the following lost re-election:
Taft: Third party challenge split the vote, though he was also somewhat unpopular.
Hoover: Worst economic crisis in our nation's history occurred.
Ford: Doesn't really count, as he had never been elected.
Carter: Poor economy, foreign affairs failures, Reagan was a much better campaigner.
Bush: Poor economy, third party splitting the vote, Clinton was a much better campaigner.
Yeah, but Truman and LBJ also tried to run for reelection, before losing in the primaries.
