... was the headline of the Chicago Tribune on the morning of November 3, 1948. The article indicated clearly that presidential candidate and governor of New York Thomas Dewey had defeated incumbent Harry S Truman for the position of President of the United States, but this was in fact not the case. The paper itself is immortalized in a photograph showing a victorious Truman holding the paper over his head, beaming at his surprise victory.
Why did this happen? In 1948, Harry Truman was losing the presidential race badly. He was very low in public opinion throughout the spring, and when he rolled into Bremerton, Iowa, on June 10, 1948, he was extremely frustrated with the way his campaign was going. Rather than delivering a sedate speech that he had delivered at other campaign stops, Truman launched into a very emotional diatribe, finally bringing one person in the crowd to stand up and shout, "Give 'em hell, Harry!" This is widely considered to be the turning point in the election, because from there on out, Truman made hundreds of whistlestops around the country, delivering rousing and inspirational speeches.
However, this lighting of a new fire under Truman didn't affect the polls much, so when George Gallup released a poll in mid-October showing challenger Thomas Dewey leading the race by more than ten percent of the vote, he declared that Dewey would win the election. Gallup's polling, at this point in American history, was accepted as near-gospel; just twelve years earlier, he had surprised many by picking FDR as the winner of the presidential race, going against the opinion of many and in the end proving to be the correct choice. As a result, many thought that Dewey had this race in the figurative bag. Undaunted, Truman continued his whistle stop campaign up until the last day of the campaign, finishing up with a run through California and other western states.
When the results started to come in from the eastern states, where the polls closed earlier, it seemed clear that Dewey had in fact won the race. He carried much of the eastern part of the United States, and on the basis of these early returns and Gallup's prediction, the Tribune opted to go ahead with a first edition announcing that Dewey had in fact won the race.
What happened next is political legend. In the western states, Truman's campaigning had done a great deal to swing a lot of third-party voters away from some strong third-party candidates to Truman. Dewey had run a conservative campaign all along, and with his perceived victory, he hadn't bothered to campaign much at all in the west. As a result, many independent voters swung to Truman in the west, handing him several states, including the big prize of California. California's electoral vote power, even then, was enough to give the election to Truman.
So on that fateful morning, Truman gave his victory speech and lofted above his head that errant copy of the Chicago Tribune, beaming wildly at a victory that most people (and according to his writings, himself included) figured would never happen.
Why is this important today? It is such a huge visual image and reminder of what can go wrong if you count your chickens before they hatch. In many ways, this event repeated itself in the 2000 Presidential race, where George W. Bush was declared the victor, then this declaration was withdrawn in fear of another Dewey Defeats Truman situation.
Perhaps more importantly, it gave some egg on the face to George Gallup, who had become overly confident in his polling procedures. As a result of this event, polling often now runs until election day in even the most clear cut of political races. The image of Truman holding the issue high above his head still can be found in several prominent places in the headquarters of the Gallup Organization, an ever present reminder of the care that needs to go into proper polling. | 
