That is a good question. FDR's doctors told him that he was a dead man with only months to live. He would never live through a fourth term. He knew that fact going into 1944. He ran anyway. There was a 60-40 chance he would have died in 1943. It is my opinion that he knew he had to manage the war and he could not risk that task to a man like Dewey until he, FDR, knew that the war could not be screwed up by his successors. I think that is a case that can be argued.