A Non-Political Political Marker, from Stefan Jovanovich
Incumbent Presidents voted out of office, either at the general election or by their party's convention (as noted):
John Adams Federalist 1800
John Quincy Adams Democrat-Republican 1828
Martin Van Buren Democrat 1840
John Tyler Whig
1844
(A 2-time loser; the Whigs failed to renominate him,
and his own party lost in the general election)
Millard Fillmore Whig 1852
(Fillmore wanted to run for re-election but his party
denied him the nomination)
Franklin Pierce Democrat 1856 (same as Fillmore)
James Buchanan Democrat 1860
Chester Arthur (?) Republican 1884
(Arthur allowed himself to be nominated at the convention; but he was
severely ill — he died 2 years later — so this really should not count)
Grover Cleveland Democrat 1888
William Taft Republican 1912
Herbert Hoover Republican 1932
George H.W. Bush Republican 1992
You could add these names too:
Harry Truman Democrat 1952
Lyndon Johnson Democrat 1968
Both were eligible to run for re-election but decline to do so because they would have almost certainly have lost.