To the best of my knowledge, Truman never really planned to run for nomination in 1952. It was his firm view that 2 terms was enough - and he had almost complete 2 terms in office. He did held off announcing it long past the decision - obviously, it helps not being perceived as a lame duck. But he never planned to run. Had he tried, I'd hate to discard him - he had won in 1948 against, probably, worse odds, hadn't he?
According to Gallup polling, Truman was the most unpopular president ever by 1952 (yes, worse than Nixon). If he ran again and was somehow renominated, there's a good chance he wouldn't have won a single state.
