Wave of Immigration
1948 - Truman runs against Thomas E. Dewey, the Republican nominee in the 1948 presidential election. The scientific pollsters, including George Gallup, all predicted that Dewey would beat Truman. (Gallup had won a dramatic bet by correctly predicting the outcome of the 1936 election.) Newspapers were so sure the pollsters were right that they printed the headline, "Dewey Beats Truman." But they were wrong.
Truman beat Dewey and the race wasn't even that close (Truman won by 3.5 percentage points). What had gone wrong? The pollsters had stopped polling a week before the election. They thought that people's votes would not change before the election. But in 1948 there were two strong independent candidates whose support eroded away in the last week. By stopping their polling too soon, the pollsters missed this shift away from the third party candidates back to the major parties. Since most of the votes shifted to Truman, he won the election.