Unknown president during 24's first season, who was either Palmer's challenger for office or the incumbent predecessor for their party.
David Palmer (D): Served one term, briefly interrupted via the 25th Amendment during, and after, Day 2 due to Palmer's perceived incompetence and illness between seasons.
We'll ignore the VP's presidency during Palmer's illness.
Palmer ended up stepping down during the debate cycle of re-election, so pretty late in his re-election campaign, pretty much allowing his opponent to win.
John Keeler (R): Served between the inauguration day after Day 3 and the events of Day 4. (About 15 months) Was seriously injured in an attack on Air Force One advancing his VP to President.
Charles Logan (R): Served between Day 4 and Day 5, the better part of two years, serving in place of Keeler. Unseated for treason during Day 5 advancing his VP to presidency.
Hal Gardner (R): Served between Day 5 and Day 6, finishing out Keeler's first term, which was handed over to Logan and then himself. Making three presidents in a single term.
Wayne Palmer (D): Elected between Day 5 and Day 6 which begins a few months into his term. He's critically injured and/or killed during the day's events advancing his VP.
Noah Daniels (D): Finished out Wayne Palmer's term and did not win reelection.
Allison Taylor (R): Sworn in during "Redemption" and was President for the rest of the series. Resigned during Day 8 due to the conspiracies in her office, advancing her VP.
James Heller (D): Elected during the election following the events of Day 8 and served until present-day in the 24 universe, advancing his VP to presidency following his assassination.
Aside from the President during Day 1, during the entire run of 24 no President has ran a full, uninterrupted, term. The closest we get is David Palmer who served a full term, setting aside the interruption caused by his cabinet and Congressional members and then illness during/after Day 2. It also means there were 9 presidents in the span of something like 18 years.
