Truman wasn’t on the ballot, whether he wanted to run or not. You don’t have to go back to ‘28 to find the situation that we are in now.
Truman was unpopular because he had just fired Gen. Douglas MacArthur from his Korean command. Ironic. Truman was attempting to chip away at MacArthur’s credibility so he wouldn’t be a threat in the ‘52 race
and instead, it ruined his own chances at reelection.
Harry Truman was on the ballot in the 1952 New Hampshire primary. He was defeated by Senator Estes Kefauver, as I noted.