President Johnson had fired his secretary of war, Edwin Stanton, without the Senate’s consent, which the recently passed Tenure of Office Act had required. Though constitutionally dubious, that law had been created precisely to prevent Johnson from obstructing justice by sacking such men as Stanton, who was helping Congress implement the Reconstruction Acts, which Johnson had also been attempting to obstruct. When President Johnson violated the law, the House voted overwhelmingly to impeach him. It then drafted 11 articles of impeachment, most of which dealt with Johnson’s violation of the statute—his apparent commission of a demonstrable crime—although he was also accused of denying the legitimacy of Congress, holding it in contempt, failing to execute its laws, and abusing his power.

Read more: How does impeachment work?

The 11 impeachment articles were submitted to the Senate, but beyond stipulating that each chamber of Congress define the rules of its own proceedings, the Constitution did not dictate what would or should happen during an impeachment trial, or even how to organize it. So Chief Justice Salmon Chase seized the day. Chase had been a political heavyweight for decades, first as a noted lawyer for fugitive slaves and then as a senator, governor, and presidential contender from Ohio before Abraham Lincoln tapped him to be Treasury secretary and, after the death of Chief Justice Roger Taney in 1864, nominated him to the Supreme Court. A founder of the Republican Party who was deeply suspicious of its progressive wing—the wing that most desired Johnson’s conviction—Chief Justice Chase was a man convinced of his own probity. He told the Senate that in case of a tie, he wanted to be able to cast a vote.

To prosecute Andrew Johnson, the House had elected seven members, called managers, who quickly argued that precedents both in England and in the United States suggested that the presiding officer, even when a member of the deciding body, had no more rights than the House did. And if that presiding officer were not a member of the body, as the chief justice was not, because he was not a senator, he should not then be allowed to vote. He could merely submit questions to the larger body, not answer them.

But that argument implicitly raised yet another issue: whether the Senate during the trial should be organized as a court of law. If the Senate operated as a court, the chief justice would be able to exercise certain leverage, such as ruling on the admissibility of evidence or the reliability of witnesses. Was this judicial meddling, as some argued? Chief Justice Chase’s conditions—that as presiding officer, he should maintain control over the proceedings—were deemed an attempt to derail the entire process. It was no secret that Chase regarded the impeachment resolution against Johnson to be absurd, even though he didn’t much like the president. What’s more, Chase had his own eye firmly fixed on the Oval Office, and his overweening ambition made his impartiality suspect, because he was said to believe that Johnson’s acquittal would serve his own presidential prospects in 1868.