What were the causes of Truman's upset victory in the election of 1948?

Answer 1

I think you should read the link I posted. It will give you a detailed
answer.

Answer 2

1948 election
The United States presidential election, 1948 is best remembered for Truman's stunning come-from-behind victory.


Truman was widely expected to lose the 1948 election, as shown, falsely, by this mistaken Chicago Tribune headline. The photograph was from the back of a train car at St. Louis Union StationAt the 1948 Democratic National Convention, Truman attempted to place a tepid civil rights plank in the party platform so as to assuage the internal conflicts between the northern and southern wings of his party. A sharp address, however, given by Mayor Hubert H. Humphrey, Jr. of Minneapolis, Minnesota, and candidate for the United States Senate—as well as the local political interests of a number of urban bosses—convinced the party to adopt a strong civil rights plank, which Truman endorsed wholeheartedly. Within two weeks he issued Executive Order 9981, racially integrating the U.S. Armed Services following World War II.[20] Truman took considerable political risk in backing civil rights, and was very concerned that the loss of Dixiecrat support might destroy the Democratic Party.

Truman's "whistlestop" tactic of giving brief speeches from the rear platform of the observation car Ferdinand Magellan became iconic of the entire campaign.[21] His combative appearances captured the popular imagination and drew huge crowds. The massive, mostly spontaneous gatherings at Truman's depot events were an important sign of a critical change in momentum in the campaign -- but this shift went virtually unnoticed by the national press corps, which simply continued reporting Dewey's (supposedly) impending victory as a certainty. It was a campaign tactic that would be most notably repeated by George H. W. Bush in 1992, who as another trailing incumbent who fought constantly with Congress, compared himself with Truman.

The defining image of the campaign came after Election Day, when Truman held aloft the erroneous front page of the Chicago Tribune that featured a huge headline proclaiming "Dewey Defeats Truman" [22].

Truman did not have a vice president in his first term. His running mate, and eventual Vice President for the term that began January 20, 1949, was Alben W. Barkley.

Answer 3

That would be hard to say because many Newspapers had already published the win for Dewy,hence his victory was a shock to most!

Perhaps some respected FDR so much they wanted to stay the course, perhaps because so many beleived he had no chance that voters went for the underdog! Perhaps people at the 11th hour decided he was more real and folklike!