As election day in 1948 drew nearer, political pollsters, pros and amateurs united in predicting victory for Thomas E. Dewey. Only underdog candidate President Truman and, as it turned out, the American electorate disagreed with this consensus.
At the end of 1947 Truman was considered certain for renomination by his party, and the Gallup Poll showed that he would easily defeat either of the two leading Republicans--Governor Dewey and Senator Taft. A get-tough policy with Russia had pushed up his popularity--the Marshall Plan, his advocacy of government action to curb high prices, his confrontation with labor leader John L. Lewis which ended in a Truman victory, all had boosted his popularity. The '47 November elections had brought favorable omens, after the devastating defeat in the Congressional elections of '46, with Democrats winning many local elections. On November 7 political commentator David Lawrence said, "The chances are less than even that the Republicans will win in 1948 no matter whom the Republicans select as their candidate."
But within four months universal pessimism had set in. Truman was considered a certain loser, and there was a strong drive to deny him renomination. Never has the political stock of a President fallen with such lightening rapidity.
Biggs, Cloene, "The Truman Election: Was 1968 A Repeat?" (1968). History Class Publications. 53.