President truman's re-election in 1948 was considered highly improbable yet he was able to secure one of the greatest electoral upsets in presidential politics. why was it improbable? what contributed to his victory?

One of the reasons why it was so improbable was that he was a Democrat, and that year, there was a 3 way split in the Democratic party (former vice president Henry Wallace's Progressive party and Strom Thurmond's Dixiecrats). Another thing was the polling data. His opponent, Senator Dewey's victory was so expected that polls stopped doing surveys about the election the September before. The night before the election, the Chicago Tribune approved a giant headline that read, "Dewey defeats Truman".
Truman started campaigning hard after Labor Day to make more than 200 campaign speeches, while Dewey was a dull speaker and didn't campaign as much. To (partly) appeal to African American voters, Truman was urged to submit a civil rights agenda to end racial segregation on public transportation, etc. The South was also traditionally Democratic, although after 1948, the Republicans took over the South. Truman had a reputation to be a fighter, and the Democrats had been in the White House for 16 years before the election of 1948. Many believed it would end there, but it didn't.