The years following his death saw the presidency of Harry Truman, the use of atomic weapons in Japan and the rise of the Cold War and anti-Communism in the United States.
"Franklin Roosevelt died at a really critical moment in American history. His vice president, Harry Truman, had only been in that office for three months, so it's worth asking what might have happened if Franklin Roosevelt had actually lived through his fourth term," Yale University history professor Beverly Gage told CNN.
Gage believes that foreign policy would largely have occurred the way it did under Truman. Roosevelt would likely have dropped the atomic bombs on Japan to end WWII and the Cold War would have risen, but perhaps more slowly than it did.
Roosevelt's major impact would have come domestically, according to Gage. After winning four elections, FDR was clearly popular with the American people. Truman, on the other hand, was a political unknown without much momentum to lead the Democratic Party or his constituents.
"The Republicans really mobilize. They won the 1946 election, and that becomes one of the moments when a new anti-Communist politics begins to emerge in the United States," Gage said. "Joe McCarthy, he comes to Washington in 1946."
Truman's distaste for the FBI and J. Edgar Hoover -- with whom FDR had a good relationship -- may have indirectly led to McCarthy's influence in another way. Hoover took the information about Soviet espionage to Congress instead of the White House, leading to the House Un-American Activities Committee and the rise of McCarthyism as a central issue of American politics.
"If Franklin Roosevelt had listened, responded, done some of the things the FBI was urging, maybe you never would have had that congressional committee moment. Maybe you never would have had McCarthyism," Gage said.
But it wasn't just Sen. McCarthy who came to Washington after 1946 -- the ripple effects of that election spanned decades.
"If Franklin Roosevelt had been in office, if he had been able to mobilize different political constituencies at home, we might not have seen both McCarthyism, but we also might not have seen John Kennedy, who was (first elected to Congress) in 1946. We might not have seen Richard Nixon," Gage said. Both future presidents won seats in the House in the key midterm.