Andrew Johnson Impeachment Trials

The galleries of this chamber were filled to capacity on March 30, 1868, as the Senate began the impeachment trial of President Andrew Johnson. Popular interest in the trial was so intense that the Senate, for the first time in its history, issued gallery passes, beginning a practice that continues to the present. Johnson, formerly a Democratic United States senator from Tennessee, had remained loyal when his state seceded from the Union, and was chosen as Abraham Lincoln's running mate in 1864 to ensure the support of the border states.

 
His accession to the presidency upon Lincoln's death in April 1865 began a period of unusually contentious relations between the legislative and executive branches, as Johnson vetoed Reconstruction legislation, fired Secretary of War Edwin Stanton in violation of the Tenure of Office Act, and ultimately became the only president in the nation's history to face an impeachment trial. The lengthy and contentious trial ended when, on May 16 and 26, the Senate voted on three of the several impeachment articles brought against Johnson. Each time the tally fell one vote short of the two-thirds majority that the Constitution requires for conviction.

Johnson was not renominated for the presidency in 1868 but returned to the Senate in 1875, serving until his death on July 31 of that year.

Source - www.senate.gov

For further reading: Benedict, Michael Les. The Impeachment and Trial of Andrew Johnson. New York: W.W. Norton & Co., 1973.
Trefousse, Hans L. Impeachment of a President: Andrew Johnson, the Blacks, and Reconstruction Knoxville: University of Tennessee Press, 1975.