From 1849 to 1850, Riddle served as a Deputy Attorney General of the United States. After losing in the election of 1844, he was elected to the U.S. House in 1850 and served for two terms from March 4, 1851, until March 3, 1855. During the 33rd Congress, Riddle was the Chairman of the Committee on Engraving. Running for a third term he was defeated in 1854 by Elisha D. Cullen. In 1860, Riddle was one of only two
slaveholders in Delaware. He owned three slaves, a 68-year-old male and two females aged 56 and 12. On February 2, 1864, Riddle was elected to the United States Senate to fill the vacancy caused by the resignation of U.S. Senator James A. Bayard Jr. He served until his death on March 28, 1867. ==Death and legacy==