He was a chair of the state delegation to the Republican Convention in Chicago that nominated Abraham Lincoln. Smithers served as secretary of state for Delaware under Governor Cannon from January 20, 1863 until November 23, 1863 when he was elected to the U.S. Congress to fill a vacancy opened with the death of Democrat William Temple. While there he served on the critical Special Committee of Reconstruction and helped turn down the efforts of Arkansas and Louisiana members to be re-admitted. He also shepherded the amendment through Congress which abolished the purchase of relief from the draft. At the Baltimore Republican Convention in 1864, he was a member of the executive committee but did not support Andrew Johnson's nomination as vice-president.