Winning with Dewey on normal mode is pretty easy if you pick the right answers. I do think the game overestimates Truman's strength though. The IRL result was more of a fluke then a referendum.
Candidate Electoral Votes Popular Votes Popular Vote %
---- Thomas Dewey 277 22,639,790 46.1% **WINNER**
---- Harry Truman 216 23,380,548 47.6%
---- Strom Thurmond 38 1,301,410 2.6%
---- Henry Wallace/Other 0 1,793,630 3.7%
I got this result right now by
- Picking Warren as my running mate
- Campaigning in New York and California
- Opposing veto on Taft-Harley bill
- Supporting civil rights, moderate on other social issues
- Regularly attacking Truman on policy (you need to play offense against an incumbent)