The conflict between President Andrew Johnson and Congress reached a climax when Johnson fired Edwin Stanton, his secretary of war. This violated the Tenure of Office Act enacted on March 2, 1867, which required the president to seek approval from the Senate before firing any government appointee confirmed by the Senate. President Johnson and Secretary of War Stanton, who supported congressional Reconstruction, were political enemies. The Tenure of Office Act was passed with the intention of protecting Stanton from removal by Johnson. In August 1867, with Congress in recess, Johnson suspended Stanton and appointed former Union general Ulysses S. Grant as acting secretary of war. Because the Senate was not present to approve or deny Johnson's action, Stanton's suspension and Grant's appointment did not violate the law.
Congress returned to Washington in January 1868 and declined to approve Stanton's removal. In response, Grant voluntarily stepped down, allowing Stanton to reclaim his cabinet position as secretary of war. Johnson then fired Stanton on February 21, 1868, without the approval of Congress. This move now was clearly in deliberate defiance of the Tenure of Office Act. The House of Representatives swiftly moved to impeach President Johnson. Impeachment is the formal accusation of misconduct against an official elected to public office. In the case of a president, the House of Representatives votes to impeach, and the Senate conducts the trial.
Impeachment proceedings began in March 1868 and lasted for two months as Johnson was tried by the Senate. The Tenure of Office Act was a focal point of the impeachment proceedings, and the constitutionality of the act was hotly debated. This was because the power over presidential appointments the act granted to the Senate was not a power granted by the Constitution. To convict an impeached president, two-thirds of Senators must enter a vote of guilty. President Johnson was acquitted by only one vote.
Grateful for his acquittal, Johnson vowed to rehabilitate his reputation. However, the damage to his political career was irreversible. Although he received a small number of votes at the 1868 Democratic Convention, he did not seek the nomination for reelection. Former Union general Ulysses S. Grant, whom Johnson had wanted to replace Stanton as secretary of war, won the Republican nomination. He would become the 18th president of the United States.