From 1945 to 1948, Harry Truman (Dem.) was the president, following the death of FDR (he died from a stroke). From 1943 until then, Thomas Dewey (Rep.) was the governor of New York. In 1944, Thomas Dewey lost to FDR in in the closest of his four presidential elections.
Virtually every prediction indicated the Dewey would defeat Truman. Many voters still viewed Harry Truman as an ineffectual president following FDR, despite his effective leadership after World War II, and sound vision in the confused postwar world. Truman also antagonized Southern Democrats with his civil rights initiatives. Most were sure Dewey would win.
Truman was showing consistently poor showings in the polls, he embarked on a "whistle-stop" campaign across the US a few weeks before the elections. He fought to win over the undecided voters, saying he was an outsider contending with a "do-nothing" Congress.
In the days preceding the vote, political analysts and polls so heavily supported Dewey that on election night, long before all the votes were counted, the Chicago Tribune published an early edition with the banner headline "DEWEY DEFEATS TRUMAN". In one of the greatest upsets in presidential election history, Harry Truman defeated Thomas Dewey, extending the streak of consecutive democratic presidential wins to five, the longest streak in the history of the party.