It's not impossible to imagine a Dewey victory. Maybe some combination of Wallace staying on the ticket, the Dixiecrats bolting, and FDR having some major health collapse:
Dewey would not be too different than Roosevelt. He'd drop the Bomb (as virtually any plausible President in 1945 would) and support aid to Europe, just as he did in OTL. Maybe he'll be seen in a weak position in 1948 but a resurgent economy will boost him back up over a Democratic candidate.