How do you ensure a Wallace Presidency (not necessarily via FDR's death)? Would taking Dewey out of the equation for the 1944 election change anything?
With great difficulty. Wallace was not really a politician. He was a businessman who was tapped for SecAg, and then for VP. He was elected VP as part of FDR's ticket, not on his own. No one voted for
him.
If FDR was forced to retire in 1944 (ill health too obvious for another term)... Wallace
might have been the Democrat nominee. Especially if FDR withdrew at the last minute. Wallace would be the obvious front runner to succeed him, and he had gained considerable support among the Democrat rank-and-file. (Though Democrat party leaders almost all thought he was a millstone.)
Still, Wallace might sweep to victory at the convention, especially if FDR gave him any kind of endorsement. (He sort of endorsed Wallace's renomination as VP, even as he was plotting with the party chiefs to replace him.)
Let's say FDR tries a similar maneuver, only instead of tabbing Truman for VP, he pushes "Assistant President" James Byrnes for President. OTL., Byrnes thought he was FDR's pick for VP, and Truman went to the convention prepared to make a nominating speech for Byrnes. ATL, he chooses Byrnes for his superb executive abilities. (Byrnes, as head of the Office of Economic Stabilization and Office of War Mobilization, practically ran the U.S. economy during WW II.)
But Byrnes had two problems, which the party chiefs felt were fatal. (OTL that's why they pushed FDR to pick someone else - i.e. Truman.) He was a die-hard segregationist from South Carolina (offensive to blacks, who had only recently shifted to the Democrats), and a lapsed Catholic (offensive to both anti-Catholic bigots
and Catholics).
FDR, sick and somewhat confused, semi-endorses Wallace publicly, then tries to maneuver Byrnes into place. While the party chiefs balk, split, and dither, Wallace wins the nomination.
Now, how to get him to win? Dewey is a much superior campaigner, IMHO, and has at least two elections under his belt. Wallace is not a complete novice, as he campaigned for FDR in 1940.
I think it's necessary that Dewey makes some dreadful blunder. IIRC, he was going to go public with accusations about Pearl Harbor that would have exposed U.S. codebreaking successes, but was talked down by Marshall. ITTL, that's unlikely because it wouldn't touch Wallace.
So what else could it be? It has to be about the war; nothing else has enough juice to swing the election.