Heading into the election of 1940, the Pendergast machine that had propelled Harry S. Truman to victory in the 1934 election had been brought to its knees as Tom Pendergast was ill and imprisoned for widespread voter fraud in the 1936 elections, in which more votes were tallied in Kansas City than its entire population. As such, Truman's connections to Pendergast going back to his 1922 candidacy for judge in Jackson County were a liability in the campaign. Missouri Governor Lloyd C. Stark, who had portrayed himself as an opponent of the machine, challenged Truman for the Democratic nomination for U.S. Senator.
Truman, who had decided to seek a second term in February 1940, overcame his Pendergast connections and defeated Stark for the Democratic nomination. Touting his experience as a World War I veteran and track record as a U.S. Senator, Truman ultimately defeated Republican candidate and former state senator Manvel H. Davis in the November general election.