Truman surprises in 1948

On election day 1948, everybody thought that Mr Dewey would be the next president of the US.    

However, the country woke up the next morning hearing that Mr Truman had been returned to office:

“In the greatest upset in presidential election history, Democratic incumbent Harry S. Truman defeats his Republican challenger, Governor Thomas E. Dewey of New York, by just over two million popular votes. 
Harry Truman was thrust into the presidency by Franklin D. Roosevelt‘s death in 1945. Approaching the 1948 presidential election, he seemed to stand a slim chance of retaining the White House. 
Despite his effective leadership at the end of World War IIand sound vision in the confused postwar world, many voters still viewed Truman as an ineffectual shadow of his four-term predecessor. 
He also antagonized Southern Democrats with his civil rights initiatives. Most were sure that Dewey would take the White House.In the last weeks before the election, Truman embarked on a “whistle-stop” campaign across the United States in defiance of his consistently poor showings in the polls. He traveled to America’s cities and towns, fighting to win over undecided voters by portraying himself as an outsider contending with a “do-nothing” Congress. 
Truman, a one-time farmer who was elevated to the pinnacle of American politics because of his reputation for honesty and integrity, won the nation’s affection, and he narrowly won a second term.”

In the days preceding the vote, political analysts and polls were so behind 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.”

No one knows for sure but I think that Mr Dewey got a little bit too confident.  On the other hand, Mr Truman worked hard for every vote.