In 1919, Clark began practicing law in St. Louis, Missouri. In the 1920s he researched and authored a biography of John Quincy Adams, and was active in politics as a campaign speaker for Democratic candidates in Missouri. In 1928 he considered running for the United States Senate seat of the retiring James A. Reed, but decided not to make the race. Clark was a delegate to the 1928 Democratic National Convention. He served again as a delegate to the Democratic National Convention of 1936. He was a delegate again in 1940, and served as a delegate to the party's national convention in 1944. In 1944, Clark made the speech nominating Harry S. Truman for vice president.
United States Senator
In the 1932 election, Clark ran for the United States Senate seat held by the retiring Harry B. Hawes, and relied on his base among veterans to defeat two other candidates for the Democratic nomination. Clark defeated Henry Kiel in the general election for the term beginning March 4, 1933. Hawes resigned on February 3, 1933, a month before his term was to end, and Clark was appointed to fill the vacancy, gaining seniority on other senators elected in 1932. Clark was re-elected in the 1938 election, and served from February 3, 1933, to January 3, 1945. In 1944, Clark was an unsuccessful candidate for renomination, losing the Democratic primary to state Attorney GeneralRoy McKittrick, who lost the general election to Republican Governor Forrest C. Donnell. Clark was chairman of the Senate Committee on Interoceanic Canals from 1937 to 1935. He was a member of the Smithsonian Institution's Board of Regents from 1940 to 1944. In April 1943 a confidential analysis by British scholar Isaiah Berlin of the Senate Foreign Relations Committee for the British Foreign Office succinctly characterized Clark as, in his strongly partisan view:
a rabid isolationist and member of the American First Committee who has steadily voted against all the foreign policies and war measures of the Administration with the exception of the reciprocal trade agreements. A member of the Wheeler-Nye- Taft coterie. An avowed Anglophobe.
Clark is perhaps most famous for declaring that Emperor Hirohito should be hanged as a war criminal on the United States Senate floor on January 29, 1944. In the same year, he was the first senator to introduce the G.I. Bill proposal in the United States Congress. When Congress began work on the G.I. Bill in 1944 it had originally expressed concern about possible misuse of the "Blue discharge". In testimony before the United States Senate, Rear Admiral Randall Jacobs strongly opposed the provision to include Veterans with Blue discharges on the grounds that it would undermine morale and remove any incentive to maintain a good service record. Senator Clark, a sponsor of the GI Bill, dismissed his concerns, calling them "some of the most stupid, short-sighted objections which could be raised". Clark went on to say:
The Army is giving Blue discharges, namely discharges without honor, to those who have had no fault other than they have not shown sufficient aptitude for military service. I say that when the government puts a man in the military service and, thereafter, because the man does not show sufficient aptitude gives him a blue discharge, or a discharge without honor, that fact should not be permitted to prevent the man from receiving the benefits to which soldiers are generally entitled.
Federal judicial service
Clark was nominated by President Harry S. Truman on September 12, 1945, to an Associate Justice seat on the United States Court of Appeals for the District of Columbia vacated by Associate Justice Thurman Arnold. He was confirmed by the United States Senate on September 24, 1945, and received his commission on September 28, 1945. His service terminated on July 13, 1954, due to his death.
In 1922, Clark married Miriam Marsh, the daughter of Wilbur W. Marsh. They were the parents of three children, Champ, Marsh, and Kimball. Miriam Clark died in 1943, and in 1945 Clark married British actress Violet Heming. The ceremony took place at the Berryville, Virginia home of Clark's sister, and President Truman served as best man.
In popular culture
Clark and other isolationist senators are referenced in the Woody Guthrie song Mister Charlie Lindbergh. Guthrie's 1943 lyrics condemn pre-World War II isolationism and advocate for leaders committed to defeating fascism.