FDR's Final Years (1943-45)
Prelude: The Third Term
Seeing World War II coming on the horizon, in 1940 President Franklin Delano Roosevelt took the unprecedented step of seeking a third consecutive term.
Not learning past lessons, Roosevelt's physicians and inner circle simply covered up FDR's health issues during his third term, giving the nation the impression that in Roosevelt, it had a leader physically and mentally up to the challenge of defeating the Axis Powers.
Winning re-election, in 1944 (and with the war in full swing), Roosevelt sought a fourth term. But suffering not only from poliomyelitis but from chronic hypertension, cardiomyopathy, malignant melanoma and heart failure, he had absolutely no business doing so.
Dumping Wallace for Truman
Democratic Party bosses knew it was imperative for the United States to have a strong, stable leader in time of war, so nominating FDR for a fourth term, even in spite of his ailments, was a given. In fact, what scared them in 1944 wasn't Roosevelt's health - it was the idea that Vice President Henry A. Wallace would succeed him if he died.
Recognizing that Roosevelt's health was so fragile and not wanting Wallace anywhere near the Oval Office, at the 1944 Democratic National Convention the powers within the party arranged to have Wallace replaced on the ticket with Missouri Senator Harry S Truman - knowing full well that by doing so, they were hand-picking the 33rd President of the United States.
The way history played out is no mystery: the Roosevelt-Truman ticket would be elected in November 1944, but FDR would die of a stroke less than three months after his inauguration. Truman, who had been left out of the loop with respect to policy and war planning as Vice President, suddenly found himself in the middle of it all.
A Little Alternate History: Acting President Wallace?
Had the Democratic Party's leadership not been scared to death at the thought of Wallace becoming President, and had there been a constitutional mechanism for doing so, it seems at least plausible that FDR could have stepped aside at least temporarily as early as mid-1943. And had he been effective in the role, it's possible the name of Harry S Truman might have slipped into relative obscurity.