Not that hard, actually. Simply have him stay as governor of Minnesota instead of signing up for the navy and quitting. While he was overseas, Dewey took over a lot of his base. So in this scenario its possible that Stassen could be the candidate in '44 (always loses to FDR), '48 (Truman still wins most likely), and '52. Ike declines to run in '52, and Stassen narrowly wins the election with the support of Dewey and Earl Warren, who will support him against Taft.