1948 was one of the great upsets in US political history. Just about every poll predicted that Republican Tom Dewey would be elected, yet President Truman narrowly beat him. Historians have attributed this to the Republicans running a bland, passive campaign as a result of their overconfidence in victory, and Truman's no-holds-barred whistle stop campaign that savaged the GOP on the stump. Further, major polling agencies actually stopped polling voters in the weeks and months the before the election, meaning that polling data at the time of the election was faulty. Many voters switched from Dewey to Truman in the final two weeks of the campaign. Since polling agencies hadn't detected this, Dewey still had a large lead over Truman on election day according to inaccurate data.
It's clear that the Republicans ran a weak campaign, while Truman and the Democrats ran a very strong one. Not only was Truman a forceful campaigner, he was able to portray himself as the man in the middle between the extremist factions of the Democratic Party and the "do nothing" Republicans. Could any other Democrat have won in 1948? Suppose that FDR had chosen either Henry Wallace or Bill Douglas, his friend and personal preference, to be his running mate in 1944. Could either one have won in 1948?