In 1924, Byrnes declined renomination to the House and instead sought nomination for the Senate seat held by incumbent
Nathaniel B. Dial, though both were former allies of the now-deceased "Pitchfork Ben" Tillman. Anti-Tillmanite and extreme racist demagogue
Coleman Blease, who had challenged Dial in 1918, also ran again. Blease led the primary with 42 percent. Byrnes was second with 34 percent. Dial finished third with 22 percent. Byrnes was opposed by the
Ku Klux Klan, which preferred Blease. Byrnes had been raised as a
Roman Catholic, and the Klan spread rumors that he was still a secret Catholic. Byrnes countered by citing his support by
Episcopal clergy. Three days before the run-off vote, 20 Catholics who said that they had been
altar boys with Byrnes published a professed endorsement of him. That group's leader was a Blease ally, and the "endorsement" was circulated in
anti-Catholic areas. Blease
won the runoff 51% to 49%. During his time in the Senate, Byrnes was regarded as the most influential South Carolinian since
John C. Calhoun. He had long been friends with President
Franklin D. Roosevelt, whom he supported for the
Democratic nomination in 1932, and made himself Roosevelt's spokesman on the Senate floor, where he guided much of the early
New Deal legislation to passage. He won an easy re-election in
1936, promising:I admit I am a New Dealer, and if [the New Deal] takes money from the few who have controlled the country and gives it back to the average man, I am going to Washington to help the President work for the people of South Carolina and the country. Byrnes also criticized those who spoke out against the interventionist policies of the Roosevelt administration by making references to Thomas Jefferson and states' rights, arguing that Jefferson was a progressive and liberal as well a "champion of the masses." Adding to his point, Byrnes asserted that Since the
colonial era, South Carolina's politicians had dreamed of an inland waterway system that would not only aid commerce but also control flooding. By the 1930s, Byrnes took up the cause for a massive dam-building project,
Santee Cooper, that would not only accomplish those tasks but also electrify the entire state with
hydroelectric power. With South Carolina financially strapped by the
Great Depression, Byrnes managed to get the federal government to authorize a loan for the entire project, which was completed and put into operation in February 1942. The loan was later repaid to the federal government with full interest and at no cost to South Carolina taxpayers. Santee Cooper has continued to serve as a model for public-owned electric utilities worldwide. In 1937, Byrnes supported Roosevelt's highly controversial
court-packing plan, but he voted against the 1938
Fair Labor Standards Act, as a
minimum wage would potentially make the textile mills in his state uncompetitive. He opposed Roosevelt's efforts to purge
conservative Democrats in the 1938 primary elections. On foreign policy, Byrnes was a champion of Roosevelt's positions of helping the
United Kingdom against
Nazi Germany in 1939 to 1941 and of maintaining a hard diplomatic line against
Japan. In this context, he denounced isolationist
Charles Lindbergh on several occasions. Byrnes played a key role in blocking
anti-lynching legislation, notably the
Costigan–Wagner Bill of 1935 and the
Gavagan bill of 1937. Byrnes said that "rape is responsible, directly and indirectly, for most of the lynching in America." Byrnes despised his fellow South Carolina Senator
"Cotton Ed" Smith, who strongly opposed the New Deal. He privately sought to help his friend
Burnet R. Maybank, then the Mayor of
Charleston, defeat Smith in the
1938 Senate primary. During the primary, however,
Olin Johnston, who was limited to one term as governor, decided to run for the Senate. Because Johnston was also a pro-Roosevelt New Dealer, but Byrnes's support was limited, Taking advice from Byrnes, Maybank decided to run for governor instead, and Byrnes made the reluctant decision to support Smith. On June 12, 1941, Roosevelt
nominated Byrnes as an
associate justice of the United States Supreme Court, and he was confirmed that same day. He served on the Court for only 15 months, from July 8, 1941, until October 3, 1942. Byrnes was the last Supreme Court Justice not to earn a
law degree. ==World War II==