Richard Nixon is the only United States president ever to have resigned the office mid-term.
Having secured a landslide re-election in November 1972, his resignation on August 9, 1974 came just eighteen months into his second term.
One consequence of Nixon’s resignation was that Gerald Ford became the first president never to have been elected by the people or the Electoral College. Ford was also the first vice-president to have been nominated by a sitting president and confirmed by the Senate.
The Washington Post on August 7, 1974:

The Washington Post on August 9, 1974:

Nixon’s Resignation – Archived Posts
- Jul 31, 1974: Watergate: Yes, It Could Happen Here
- Aug 08, 1974: Nixon Resigns: ABC, CBS And NBC Television Coverage
- Aug 08, 1974: Nixon’s Resignation Speech
- Aug 08, 1974: Nixon Recalls His Last Full Day In The White House
- Aug 09, 1974: Memorandum To Jaworski On Prosecuting Nixon
- Aug 09, 1974: Nixon’s Final Remarks To The White House Staff
- Aug 09, 1974: Nixon’s Resignation Letter
- Dec 22, 1996: The Speech That Might Have Been: Nixon Refuses To Resign