Harry S. Truman is something of an anomaly in American politics. He served in World War I as an artillery officer, and then he went on to own a haberdashery (a store selling men's clothing and accessories).
Hence the nickname "Harry the Haberdasher."
Eventually bored with selling suits and cufflinks, Truman decided to join the Democratic Party and run for judge of a County Court. This fateful decision got Truman involved in party machine politics, as he couldn't have hoped to run (much less win) without the aid of Kansas City political boss Tom Pendergast. This campaign began a long and fruitful relationship between the two men, which goes to show you that machine politics was a lot like Big Tobacco: get 'em early, and they're yours for life.
After his service as a judge, Truman got more ambitious. He wanted to be governor, or at least in Congress, so Pendergast backed him in the Missouri Senatorial race of 1934. He wasn't Pendergast's first choice, but he won anyway due to the powerful grinding of Pendergast's political machine. In return, Truman deferred a lot of his decisions to the man who got him elected.
Of course, if you asked Truman, he'd say that he voted with his conscience and that he was his own man, but this was patently untrue. Truman was in so deep with the party machine that he was jokingly known as "the Senator from Pendergast" rather than from Missouri. This relationship would launch his career as well as end it. While he managed to break out of this mold somewhat in the late 1930s and early '40s by chairing an investigative committee into military waste and war-profiteering, ultimately his reputation was staked to that of the party bosses.
Truman was a compromise candidate for Vice President in the 1944 election, and earned the nomination despite the more popular (and liberal) Henry Wallace polling better among voters. This turned out to be a spectacular measure of foresight on the part of the party bosses, as FDR died shortly afterwards and Truman became president. Pretty poor timing on FDR's part, as World War II hadn't quite been wrapped up yet. The fateful decision to use atomic weapons fell to Truman, who opted for their use rather than a long, drawn-out invasion which was estimated to cost a quarter of a million American lives. Truman defended this decision throughout his life, and it superseded his rampant corruption as his legacy.
Truman never had FDR's pizzazz. Burdened with the difficult jobs of transitioning the economy towards peacetime, dealing with conflicts between labor and management, combating the rising threat of communism, and aiding the recovery of Europe, Truman's popularity took a serious beating. During the Democratic nominating convention, Truman tried to strengthen his position by laying out a strong civil rights platform, but all that did was to make most of the Mississippi and Alabama delegates storm out of the convention. Truman managed to hang on and secure the nomination anyway.
In the general election, the odds were on Truman's opponent, Thomas E. Dewey, the Republican governor of New York. Gallup and Roper polls had Dewey ahead by 10-15 points.
But Truman took to the highways (well, the railways) in his campaign. In what was known as the "Whistle Stop Campaign," he chartered a train and took his case straight to the American people, speaking from the back of the train at railroad stations all over the country. He hoped his "common man" image would win people over.
On election Eve, the Chicago Tribune printed its now famous headline: "Dewey Defeats Truman", making the Gallup and Roper polls and the Chicago Tribune headline the most epic fails in election history.
When the Korean War broke out, Truman made the bewildering move of dismissing General Douglas McArthur from leading the war because MacArthur wanted to bomb China. Firing the wildly popular and insanely decorated MacArthur was probably enough to turn Americans against Truman, but on top of that was Senator Joe McCarthy and his accusations that Truman's State Department was crawling with Communists. On top of that were the investigations into corruptions and scandals by the Dems' own Estes Kefauver.
It's not hard to see why Truman decided not to run for a third term and left office with one of the lowest approval ratings of all time.
Truman has a mixed legacy: his supporters called him a tough, no-nonsense leader who was willing to make hard choices, and his detractors said he was ineffectual, inadequate, and indecisive on domestic issues. What's maybe most ironic about Truman is that he left office highly criticized as corrupt—a major talking point for the Republicans in the 1952 election and one which Nixon personally hammered at incessantly. Just twenty years later, he was cast as a folk hero and compared favorably to the Nixon administration, whose popularity was tanking.
Why? Corruption and a lack of personal integrity, of course.