After presiding over the impeachment trial of President Clinton, Chief Justice William Rehnquist summed up his experience with a line from Gilbert and Sullivan. He declared, “I did nothing in particular, and did it very well.” In some ways, Rehnquist had the preferred experience of the “presiding official” in assuming the level of involvement of a ficus plant.

Chief Justice John Roberts may not have the same luxury with the trial of President Trump. With the leaking of parts of a book manuscript written by former national security adviser John Bolton, Roberts may have to rule on key, and even determinative, matters. In doing so, Roberts may have to look not to Rehnquist but to Chief Justice Salmon Chase, who presided over the 1868 impeachment trial of President Johnson. Rehnquist had the luxury of doing nothing in particular in that Senate trial with an outcome preordained by the mutual agreement of the two parties. Roberts is in the midst of a trial of a very different sort, with an array of difficult questions and no Senate consensus. Within days, senators may learn the meaning of his “rules of order” in an impeachment trial that could spin out of control.

The Bolton leak puts Roberts in a perfect nightmare. Under Senate rules, Roberts “may rule on all questions of evidence, including, but not limited to, questions of relevancy, materiality, and redundancy of evidence and incidental questions.” Indeed, that would cover a motion for determining the relevancy of witnesses as well as the decisions on allowable questions for those witnesses. But his decision could be overturned by a majority of senators. It is not true, as argued in a New York Times column, that such decisions would take a two-thirds vote to overturn him. However, he will be in largely uncharted waters with only two predecessors to guide him.

Rehnquist, by his own words, proudly did “nothing in particular” in the impeachment trial, while Chase did a lot and wanted to do a lot more. The two could not be more different. Roberts is a buttoned down conservative Republican who abhors the court playing transformative roles in political areas. Chase was a politician before, and some critics would say during, the Johnson impeachment. He had a robust view of his role as presiding officer. Indeed, if Chase were to choose a Gilbert and Sullivan quote, it would have been quite different than the one picked by Rehnquist. Chase would more likely have quoted the character Yum Yum, who said, “I mean to rule the earth, as he the sky. We really know our worth, the sun and I.”

Chase was not only a former politician, serving as Ohio governor and as a senator, but he had future ambitions. He lost the presidential nomination to Abraham Lincoln and was not shy about mixing his political and judicial interests. Before sitting as the judge during the trial of former Confederate States president Jefferson Davis, Chase had feared that convicting Davis would undermine his chances with the south in the next election. Chase solved that problem by telling a lawyer for Davis to argue that Davis could not be convicted of treason because he had forfeited his own citizenship. When the lawyer made that argument, Chase quickly dismissed the case.

Chase also faced another dilemma. If Johnson was removed with the vice presidency vacant, Senate Pro Tempore Benjamin Wade, an Ohio senator and a political rival of Chase, would become president. Chase made no secret he favored the acquittal of Johnson. Chaise claimed that, since the Senate had fashioned itself as a court, he could vote on the merits as a member of that court in the event of a tie. This was a highly contestable position and, in my opinion, just wrong as a matter of constitutional law.

Fortunately, however, it did not come to that. Chase was sustained in his evidentiary rulings and Johnson acquitted by one vote. In one dispute, a challenge was raised over questions for General William Sherman on the grounds that it would improperly recall a witness. Chase shut down the objections, declaring, “The chief justice thinks it is entirely competent for the Senate to recall any witness.” Ironically, while less inclined to play a more dominant role than Chase, Roberts may have it forced upon him.

He may be asked to rule on the relevance of witnesses like Hunter Biden or the Ukraine whistleblower, as well as issuing subpoenas for people like Bolton. Democrats are eager to force a ruling from Roberts to prevent the White House from going to a district court, thus starting a longer litigation process. However, once they cross the rubicon on witnesses, they could find Biden and the whistleblower waiting on the opposite shore. It is more likely that Roberts will defer to the Senate majority on the need for such witnesses, rather than impose his judicial will on the course of the trial.

On Bolton, it might get even more sporty for Roberts. If Bolton is called to testify in the impeachment trial, he would appear in a deposition where the White House could object, without cameras, on grounds of immunity and executive privilege. If Roberts were to rule on it, he could well reject the immunity claim, as he should in my view, but he would have to grapple with executive privilege. A communication between a president and his national security adviser is a core protected area under Supreme Court precedent. This is a noncriminal matter, unlike the case of President Nixon in which an independent counsel was seeking evidence. However, it is an impeachment. Roberts is likely to allow the testimony, but he would need to look at each area and communication for any possible privilege bars.

If Roberts starts to rule against either party, he certainly knows that his role will become increasingly controversial in the trial. The decision in Bush versus Gore was bad enough for the Supreme Court, which was criticized for effectively picking the next president. Roberts could be accused of effectively removing the current one. He may then be the subject of another fitting Gilbert and Sullivan line, “No one can have a higher opinion of him than I have, and I think he is a dirty little beast.”

Jonathan Turley is the Shapiro Professor of Public Interest Law for George Washington University and served as the last lead counsel during a Senate impeachment trial. He testified as a witness expert in the House Judiciary Committee hearing during the impeachment inquiry of President Trump.