Who was the last sitting president to lose the bid or re-election for a second term?

In 1980, Jimmy Carter became the first since 1932 to lose his re-election bid.

In 1976, Gerald Ford became the first incumbent to lose since 1932. However, he was not running for re-election since he was not elected the first time.

Lyndon Johnson chose not to run in 1968 and so avoided the likelihood of losing a reelection bid.