A power broker in the St. Louis Democratic Party allied with Senator Bennett Clark, Hannegan helped save the political career of Harry S. Truman in 1940 following the tax fraud conviction of Truman's ally, Kansas City boss Tom Pendergast. Hannegan supported Truman for re-election to the U.S. Senate when he was challenged in the Democratic primary by Governor Lloyd C. Stark and Maurice M. Milligan, who both sought credit for bringing down Pendergast. Truman re-won the seat with increased support in St. Louis, particularly from Catholic neighborhoods in which Hannegan wielded considerable influence. In his second term, Truman achieved national prominence by chairing a Senate committee investigating government waste in defense contracts. When Franklin D. Roosevelt offered Truman the position of DNC chairman, Truman declined but recommended Hannegan.