When the Senate impeachment trial of President Trump opens, the man in the center chair will be Chief Justice John Roberts. His role is spelled out in the Constitution.

It says that the chief justice "shall preside" over the Senate trial of a president. Those who envision a powerful role for Chief Justice Roberts point to Senate rules that would allow him to make decisions on "all questions of evidence." In theory, at least, that might enable Roberts to rule on motions from the House managers seeking to compel testimony from White House aides, like former national security adviser John Bolton, and others who, until now, have been blocked by Trump from testifying.

Yet under Senate rules, it is the senators themselves who have the first and last word. They establish the procedures for the trial and can, by majority vote, overturn any of the chief justice's rulings. The extent of the chief's powerlessness was driven home to Chief Justice William Rehnquist at the beginning of President Clinton's Senate impeachment trial in 1999.

When Rehnquist asked Senate Sergeant at Arms James Ziglar how to turn on his microphone, Ziglar replied, "You don't. We control that."

"I don't know if shock is the right word," Ziglar recalls, but Rehnquist certainly had "a sense of dismay" that even though he was going to be the presiding officer at the trial, "he really had absolutely no control of his courtroom."

The Clinton trial was conducted in a far different time. Republican Majority Leader Trent Lott and Democratic Minority Leader Tom Daschle worked closely to conduct the trial in a dignified way, and the rules for the trial were ultimately established by consensus and a unanimous Senate vote. President Clinton, while controversial, had never attacked the Supreme Court or the chief justice personally, and the high court membership itself had remained stable for five years, without change.

None of that is true today.

Trump's relationship with Chief Justice Roberts has been rocky at best. During the 2016 presidential campaign, Trump called Roberts a "disgrace," a "disaster," and a "nightmare," and castigated him repeatedly for casting the deciding vote to uphold the Affordable Care Act.

Although the president and Roberts have been cordial in their personal interactions in public since then, Trump has repeatedly cast aspersions on the integrity of federal judges for decisions he disagreed with, a practice that finally prompted a rare rebuke from Roberts, and of course, a snarky tweet from Trump firing back at the chief justice.

Roberts, a conservative appointed to the court by President George W. Bush 15 years ago, has tried mightily to convey an image of the court as apolitical. As he put it last year: "We do not speak for the people, but we speak for the Constitution. Our role is very clear. We are to interpret the Constitution and laws of the United States and ensure that the political branches act within them. That job obviously requires independence from the political branches."

But that image has proved harder to maintain, as the court, with the addition of two Trump-appointed justices, has swerved to the right, with the promise of an even more dramatic conservative turn in the near future.

Even as Roberts will be presiding over the Senate trial, he and fellow justices are hearing a series of blockbuster cases this term on issues as diverse as abortion, LGBTQ employment rights, a major immigration case involving the potential deportation of 700,000 "DREAMers," and even a series of cases in which Trump is seeking to block subpoenas of his financial records.

With that menu dominating his day job, it is unlikely that Roberts will want to insert himself as a major player in the impeachment trial.

Rather, he is likely to use as his role model the man he once clerked for, Chief Justice Rehnquist. Rehnquist was no stranger to the subject of impeachment. He had written a book about the 1868 impeachment trial of President Andrew Johnson, and just five years before presiding over the Clinton trial in the Senate, he had written the Supreme Court's decision declaring that under the Constitution the courts have no power to dictate the Senate rules in an impeachment proceeding.

As the presiding officer at the Clinton trial, Rehnquist relied heavily on the Senate parliamentarians for guidance. "I think he enjoyed sort of being part of history in that sense. But he understood he had a very limited role," says Robert Schaffer, one of Rehnquist's law clerks back then.

Washington University law professor Neil Richards, another former clerk, recalls that "no matter how boring or tedious the proceedings," the chief justice knew that even that limited role was important.

On a few occasions, Rehnquist did make rulings, none of them overturned by the senators. When Democratic Sen. Tom Harkin of Iowa objected to one of the lawyers referring to the senators as jurors, Rehnquist replied this way: "The chair is of the view that the objection ... is well taken, that the Senate is not simply a jury; it is a court in this case. Therefore counsel should refrain from referring to the senators as jurors."

Former Senate Sergeant at Arms Ziglar, who had once served as a Supreme Court law clerk himself, remembers that Rehnquist was often"grumpy" at the unplanned breaks that were supposed to be brief but often lasted for one or two hours. So Ziglar took the chief justice to lunch.

"And I said to him, 'Chief, what can I do to make you less grumpy?' And he said, 'Well, I love cookies-and-cream ice cream ... and chocolate chip cookies.' " After that, whenever there was a break, Ziglar would trigger the call for ice cream and cookies, and it seemed to work.

Not enough, though, to deal with the boredom of those unplanned breaks. On one occasion, Ziglar found the chief, with his three clerks, sitting in shirtsleeves in the ornate gilded room assigned to them during the trial. They were playing poker.

"I looked at him and smiled," recalls Ziglar. "You may not know this," Ziglar gently told the chief justice, "but I'm also the chief law enforcement officer in the Senate," and gambling is illegal here.

Ziglar told the card players he needed to do something for a few minutes but would be back shortly. Then he went outside the room, waited, and came back in five minutes.

"When I walked back in, the cards were still on the table," he says with a chortle, "but instead of nickels and dimes and pennies, there were little pieces of paper on which was written, one, five, and 10."

Chief Justice Roberts may find the going a lot less boring in the upcoming Senate trial, especially if the rules are often in dispute. Word is that he, like Rehnquist before him, has asked that the trial begin on Court days at 1:00 in the afternoon so that Roberts can preside over oral arguments at the court in the morning.

In addition, the chief justice will have a very full plate of work, including writing his share of opinions, doing the administrative work of the court, and his other duties as essentially the head of the federal judiciary.

He likely will aspire to Rehnquist's puckish evaluation of his own role in the 1999 Senate trial. Borrowing a line from Gilbert and Sullivan's play Iolanthe, Rehnquist opined, "I did nothing in particular and did it very well."

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