Thanks in no small part to the strain placed on his shoulders by the events of World War II, throughout the final two years of his life President Franklin Roosevelt is reported to have suffered from a variety of life-threatening ailments:  chronic hypertension, cardiomyopathy, malignant melanoma, heart failure, high blood pressure and others.  Like Wilson however, FDR's physicians and those in his inner circle simply covered up his health problems, giving the nation the impression that in Roosevelt it had not only a charismatic leader, but one that was physically and mentally up to the challenge of defeating the Germans, Italians and Japanese.  Having successfully kept Roosevelt's poliomyelitis out of the public eye for decades, his inner circle was well-versed in health-related cover ups..

Seeing war on the horizon in 1940, FDR took the unprecedented step of seeking a third consecutive term.  In 1944 with the war in full swing, he ran for a fourth term - but had absolutely no business doing so.  By then, Roosevelt's body was a veritable time bomb, and his physicians and most of the Democratic Party hierarchy knew it.  As they felt it was more important that America have strong, continuous, stable leadership in a time of war, the party bosses decided to run FDR again anyway. 

At the 1944 Democratic National Convention and with Roosevelt's tacit approval, party leaders dropped incumbent Vice President Henry A. Wallace (pictured at middle right) - who was viewed by many as an outright Communist sympathizer - in favor of Missouri Senator Harry S Truman, knowing they were likely choosing the next President of the United States.

Roosevelt won a fourth term, but would die of a stroke less than three months following his inauguration.  Had the nation an effective presidential succession and disability law however, it seems at least plausible that Wallace could have acted as President during the final portion of FDR's third term.  And had Wallace been effective and run for the White House himself in 1944, it's entirely possible that the name of Harry Truman would've slipped into relative obscurity forever.