In 1934, Boss Tom Pendergast of Kansas City needed a compromise candidate for the Democratic nomination for the US Senate, and Truman was probably chosen, as an honest politician, to put a good face on the machine. With his patron Pendergast in prison by 1940, Truman's prospects for re-election weren't good, but he managed to squeak to victory because he had two opponents in the primary who split the vote. In 1944, under circumstances that still aren't quite clear, he become the vice presidential nominee with FDR for his fourth term -- tantamount to nomination for president, though an unspoken one.
Though the people of the United States thought enough of President Truman to re-elect him in 1948 despite polls and educated opinion that said it wouldn't happen, his presidential reputation wasn't as high in office as it would be later. These days he tends to be accorded above-average or near-great status as a capable successor of FDR, who had very big shoes, during a triumphant but difficult time.