Andrew Johnson was a lifelong Democrat and slave owner who won a place alongside Abraham Lincoln on the 1864 Republican ticket in order to gain the support of pro-war Democrats. Johnson was fiercely pro-Union and had come to national prominence when, as a Senator from the important border state of Tennessee, he denounced secession as treason. In May, 1868, the Senate came within a single vote of taking the unprecedented step of removing a president from office. Although the impeachment trial of Andrew Johnson was ostensibly about a violation of the Tenure of Office Act, it was about much more than that. Also on trial in 1868 were Johnson's lenient policies towards Reconstruction and his vetoes of the Freedmen's Bureau Act and the Civil Rights Act. The trial was, above all else, a political trial.
Douglas O. Linder,
The Impeachment Trial of Andrew Johnson,
Available at: https://irlaw.umkc.edu/faculty_works/856