November 19, 1856 |- | nowrap | Maine(1) | nowrap | Hannibal Hamlin (D) | Resigned January 7, 1857, to become Governor of Maine.Successor was elected January 16, 1857. | nowrap | Amos Nourse (R) | January 16, 1857 |- | nowrap | Delaware(2) | nowrap | Joseph P. Comegys (W) | Appointment expired January 14, 1857, upon successor's election. | nowrap | Martin W. Bates (D) | January 14, 1857 |- | nowrap | Iowa(3) | nowrap | James Harlan (FS) | Owing to irregularities in the legislative proceedings the Senate declared the seat vacant January 5, 1857.Incumbent was subsequently re-elected January 29, 1857, to fill the vacancy caused by his ouster. | nowrap | James Harlan (R) | January 29, 1857 |} House of Representatives Replacements: 6 Democrats: 2 seat net loss Opposition: 4 seat net gain Deaths: 4 Resignations: 5 Contested election: 1 Total seats with changes: 10 |- | | Vacant | style="font-size:80%" | Rep-elect Lyman Trumbull resigned in previous congress after being elected to the US Senate | | James L. D. Morrison (D) | Seated November 4, 1856 |- | | | John G. Miller (O) | style="font-size:80%" | Died May 11, 1856 | | Thomas P. Akers (A) | Seated August 18, 1856 |- | | | Thomas H. Bayly (D) | style="font-size:80%" | Died June 23, 1856 | | Muscoe R. H. Garnett (D) | Seated December 1, 1856 |- | | | Laurence M. Keitt (D) | style="font-size:80%" | Resigned July 15, 1856, after being censured in his role in the assault on US Senator Charles Sumner. He was subsequently re-elected to fill the vacancy | | Laurence M. Keitt (D) | Seated August 6, 1856 |- | | | Preston Brooks (D) | style="font-size:80%" | Resigned July 15, 1856, after assaulting US Senator Charles Sumner. He was subsequently re-elected to fill the vacancy | | Preston Brooks (D) | Seated August 1, 1856 |- | | | James C. Allen (D) | style="font-size:80%" | House declared on July 18, 1856, he was not entitled to seat. He was subsequently re-elected to fill the vacancy | | James C. Allen (D) | Seated November 4, 1856 |- | | | José M. Gallegos (D) | style="font-size:80%" | Contested election July 23, 1856 | | Miguel A. Otero (D) | Seated July 23, 1856 |- | | | John W. Whitfield (D)