Have him not be a useless moderate hero.
So you would have suggested that Dewey move to the right for the campaign? I think that would have been a bad move, since conservatives were already solidly behind him and that would only alienate many moderate/liberal voters who still remembered the Hoover's conservatism and his inept response to the Great Depression.
Not move to the right, but stand for something. He'd give these long speeches about literally nothing.
"Remember, you're future is still ahead of you"