in 1856 and 1857 during the 34th and 35th Congresses. 34th Congress |- ! | Laurence M. Keitt | | Democratic | 1853 | Incumbent resigned July 15, 1856, following the caning of Charles Sumner.Incumbent re-elected July 29, 1856. | nowrap | |- ! | Preston Brooks | | Democratic | 1853 | Incumbent resigned July 15, 1856, following the caning of Charles Sumner.Incumbent re-elected July 29, 1856. | nowrap | |- ! | John Gaines Miller | | Whig | 1850 | | Incumbent died May 11, 1856.New member elected August 4, 1856.Know Nothing gain.Winner was not candidate for full term, see below. | nowrap | |- ! | William Alexander Richardson | | Democratic | 1847 | | Incumbent resigned August 25, 1856, to run for governor of Illinois.New member elected November 4, 1856.Winner was not candidate for full term, see below. | nowrap | |- ! | James C. Allen | | Democratic | 1852 | Incumbent resigned July 18, 1856, amid election challenge.Incumbent re-elected November 4, 1856.Winner was not candidate for full term, see below. | nowrap | |- ! | colspan=3 | Vacant | | Rep.-elect Lyman Trumbull (D) was later elected U.S. senator and took office March 4, 1855.New member elected November 4, 1856.Democratic hold.Winner was not candidate for full term, see below. | nowrap | |- ! | Thomas H. Bayly | | Democratic | 1853 | | Incumbent died June 23, 1856.New member elected November 4, 1856.Democratic hold. | nowrap | |- ! | John Wilkins Whitfield | | Democratic | 1854 | Seat declared vacant August 1, 1856.Incumbent re-elected November 5, 1856. | nowrap | |- ! | James Meacham | | Whig | 1849 | | Incumbent died August 23, 1856.New member elected December 1, 1856.Republican gain. | nowrap | |} 35th Congress |- ! | colspan=3 | Vacant | | Rep.-elect James S. Green (D) was later elected U.S. senator and took office January 12, 1857.New member elected August 3, 1857.Democratic hold. | nowrap | |- ! | James Lockhart | | Democratic | 18511852 1856 | | Incumbent died September 7, 1857.New member elected October 7, 1857.Democratic hold. | nowrap | |- ! | Samuel Brenton | | Republican | 1854 | | Incumbent died March 29, 1857.New member elected October 7, 1857.Republican hold. | nowrap | |- ! | John Gallagher Montgomery | | Democratic | 1856