Sitting Presidents who have lost in the last 100 years:
Taft, challenged for the nomination by Teddy Roosevelt, beaten by him and Wilson in the general.
Hoover, only a weak primary challenger, no 3rd party candidate to speak of. The exception.
Carter, strong primary challenge from Teddy Kennedy, moderate 3rd party challenger (Anderson.)
Bush I, moderate primary challenger (Buchanan), strong 3rd party candidate (Perot.)
More striking is how many sitting Presidents won re-election in that time: Wilson, Coolidge, FDR (3 times), Truman, Eisenhower, Johnson, Nixon, Reagan, Clinton & Bush II.