Overview
The plan Booth set for the night of 14 April 1865 was not a single murder. Lincoln was to be shot at Ford's Theatre, Secretary of State William Seward stabbed at his home, and Vice President Andrew Johnson killed at the Kirkwood House hotel. Two of the three attacks happened. Seward was stabbed repeatedly and survived. George Atzerodt, assigned to Johnson, spent the evening drinking and left without going upstairs.
Johnson became president the following morning.
The Card
Earlier that same afternoon, Booth had called at the Kirkwood House and, finding Johnson out, left a calling card. Its text was read into the record at the conspirators' trial: "Don't wish to disturb you. Are you at home? J. Wilkes Booth."
There are innocent readings. Booth may have been checking whether Johnson was in the building so that Atzerodt would know where to find him. He had a slight prior acquaintance with Johnson's private secretary. But the card exists, it is in Booth's hand, and it was left at the residence of the man who was hours away from inheriting the presidency, by the man about to vacate it.
The Theory
Radical Republicans in Congress took it seriously, and their motive for doing so was political as well as evidentiary. Johnson, a Tennessee Unionist Democrat placed on the ticket for balance, had turned out to oppose almost everything they wanted from Reconstruction. He vetoed the Civil Rights Act and the Freedmen's Bureau bill, opposed the Fourteenth Amendment, and issued pardons to former Confederates in volume.
Representative James M. Ashley of Ohio was the theory's principal advocate. His view was that Johnson had known of the plot, had benefited from it by design, and that the pattern of his presidency since was evidence of the arrangement. On 7 January 1867 the House voted to direct the Judiciary Committee to inquire into Johnson's conduct, and the assassination allegation was among the matters within its scope.
What the Inquiry Produced
Very little, and what it produced discredited it. Ashley's case leaned on Charles Dunham, a professional witness operating under the alias Sanford Conover, who had already supplied fabricated testimony implicating Confederate leaders in the assassination and who was convicted of perjury. Having built on that foundation, the assassination charge collapsed.
The committee voted against recommending impeachment in June 1867. It reversed itself on 25 November 1867, recommending impeachment by five votes to four on grounds that had by then largely shifted away from the assassination and toward Johnson's conduct in office. The full House rejected that report in December.
Johnson was impeached the following February, but on entirely different ground — his removal of Secretary of War Edwin Stanton in defiance of the Tenure of Office Act. He was acquitted in the Senate by a single vote.
What It Established
The claim failed for a reason that recurs: its strongest evidence, the card, is genuinely odd and genuinely unexplained, and its advocates chose to reinforce it with a witness who invented things. Once Dunham was exposed, the odd fact could no longer be examined on its own terms.
What remains is a matter of record rather than of theory. Booth called at the vice president's hotel that afternoon and left a note. The man detailed to kill Johnson did not make the attempt. And Congress, within two years, thought both facts warranted a formal inquiry into whether the sitting president had helped arrange the death of his predecessor.