Du Pont retired from business activities in 1915 and became involved in
Republican Party politics, as a member of the
Republican National Committee from 1908 until 1930. It was largely under his leadership that the Union Republicans and Regular Republicans came back together and built the modern Delaware Republican Party. Du Pont was appointed to the U.S. Senate on July 7, 1921, to fill the vacancy caused by the resignation of U.S. Senator
Josiah O. Wolcott. During this term, he served with the Republican majority in the 67th Congress. However, he lost his bid to complete the term in a special election on November 7, 1922. On the same day he also lost his bid for the full term to follow, in both instances losing to Democrat
Thomas F. Bayard, Jr., a Wilmington lawyer who was married to one of du Pont's cousins. Du Pont was elected to the U.S. Senate in 1924, defeating the incumbent Republican U.S. Senator
L. Heisler Ball for the nomination, and going on to defeat Democrat
James M. Tunnell, a
Georgetown, Delaware lawyer. During this term, he served with the Republican majority in the
69th and
70th Congress, until health problems caused him to resign. In all, du Pont served two separate terms, one from July 7, 1921, until November 21, 1922, during the administration of U.S. President
Warren G. Harding, and the other from March 4, 1925, until December 9, 1928, during the administration of U.S. President
Calvin Coolidge. In 1927, he introduced a bill proposing the construction of "a coast-to-coast superhighway, five hundred feet wide, as direct as possible from the Atlantic to the Pacific coast". However, this proposed superhighway never came to be, due in part to the economic pressures of the Great Depression. The later years of his life were marked by his implication in the
Teapot Dome scandal, and by lawsuits over various Florida real estate deals. In 1922, du Pont and his wife donated property on the Chesapeake Bay in Maryland to the
Del-Mar-Va Council of the Boy Scouts of America for development of what is now known as Camp Rodney. ==Death==