Why did Truman won the presidential election of 1948?

The opinion polls were incorrect. Many pundits had seen the polls and thought that Truman was going to lose, but failed to notice a large swing before the election. For example, the economy had been in a recession the previous year, but was improving by the time of the election, especially in rural areas. Harry Truman went on a frantic campaign around the country, and found he was still able to drum up huge support wherever he went.

Furthermore, he attacked his opponent, Thomas Dewey, portraying him as being indecisive. Dewey played it safe, believing the best way to win was to make sure he did not lose his popular, so he avoided many controversial issues, while Truman appeared aggressive and determined. The Republican-controlled Congress was almost as unpopular as Truman was, so he lambasted the "Do Nothing Congress" in his speeches and associated it with Dewey. It didn't matter that Dewey was one of the more liberal Republicans.