Give 'em Hell Harry!

Harry S. Truman became president upon the death of Franklin Roosevelt. His first term in office saw the end of World War II and the beginning of the Cold War. Truman took firm steps to stop Soviet expansion and ensure Democratic governments would not fall to the left. As 1948 approached, he considered stepping down in favor of Dwight Eisenhower. Ike turned down the Democratic and Republican nominations for president, so Truman decided to run himself.
Truman angered many in his own party. The left did not like the strong anti-communist steps the president took. They despised Truman's hard line stance against the Soviets. So, they bolted and formed the Progressive Party. This was a precursor to the modern Democratic Party. The Progressives nominated former Vice President and mystic Henry Wallace. Their platform called for making nice with Stalin and attacked the Marshall Plan (which saved Europe from starvation).
If losing the looney left was not bad enough, Truman also lost the South. Southern Democrats liked to oppress African-Americans. Truman stood for Civil Rights and desegregated the military. The South bolted and nominated Strom Thurmond of South Carolina for president. The Dixiecrats stood for segregation and white supremacy.
The Republicans were delighted. The Democrats had split into three camps. They figured they had it won. All they had to do was not lose. The GOP nominated Thomas Dewey of New York. Dewey gave FDR a run for his money in 1944. The Republicans employed the same type of strategy the Detroit Lions use at the end of ballgames. The idea was to not say or do anything to lose the election.
While Dewey grew complacent with his conservative campaign strategy, Truman went on the attack. He attacked the GOP mercilessly and effectively. His whistlestop campaign tours are now legendary. Thousands flocked to hear him speak. At one stop, a voter yelled "Give 'em Hell Harry!" Truman attacked a "Do-Nothing Congress" for being obstructionist and promised a Fair Deal for all. In reality, there was little difference between Dewey and Truman on issues.
Everyone thought Dewey would be president. The Democrats split into New Deal, New Left, and Dixiecrat camps. Truman's base would be split three ways and Dewey would cruise to victory. Truman won. How did this happen? Dewey failed to respond to Democratic attacks and failed to run an aggressive campaign. Voters saw Truman and had his message beat into them. Truman got 49% of the vote to Dewey's 45%. It was the greatest presidential upset ever.
In his second term, Truman would become bogged down in Korea and end up being the least popular incumbent in the modern era. Dewey would serve three terms as New York governor and then withdraw from politics. Henry Wallace would go back to his farm. Strom Thurmond would be a United States Senator for the rest of the century and live to 100 years old. After his death, news broke that Thurmond had fathered a biracial daughter.