Although New York Governor Thomas E. Dewey was fully expected to win the 1948 presidential election, Democratic incumbent Harry S. Truman defeated him by >2 million popular votes.

 

 

Wikipedia

 

Prior to the vote, political analysts, newspaper editorials and voter polls were so convinced of a Dewey win that on election night, long before  the final votes were counted, the Chicago Tribune published an early edition with the headline “DEWEY DEFEATS TRUMAN.”