Something I just found out that I never realised before was that Harry Truman actuatly struggled when he sought a second term in 1940. He only won the primary by 8000 votes, likely benefiting from split opposition. He then only won in the general by a slim margin.
So what would have happened had the campaign went differently, and Truman lost either the primary or the general? Would he have wanted to/been able to come back into politics later on? And more importantly, who would FDR pick in 1944 when he needed a new running mate?