WASHINGTON - Break out the umpire trope one more time: If Chief Justice John Roberts is called upon to preside over a Senate impeachment trial of President Donald Trump, it will be the most high-profile and likely unwelcome test of his skills as neutral arbiter.

Roberts steps into a precarious spot after a spat with Trump last year over the president's derogatory remarks about federal judges and sharp criticism from Democrats and Republicans in the Senate about Supreme Court rulings.

It is an ill-defined role, dictated by rules and precedents developed by the Senate. Majority Leader Mitch McConnell, R-Ky., briefly described the process Tuesday and offered a prediction.

"If it were today, I don't think there's any question, it would not lead to a removal" of Trump, he told reporters.

The trial would be an unfamiliar moment in the spotlight for Roberts, 64, whose evocation of the judge as impartial umpire drew praise at his 2005 confirmation hearing. His nomination by President George W. Bush capped off a career as a Republican operative in the White House, a lawyer with many appearances before the Supreme Court and an appellate judge.

Trump's impeachment proceedings would be the first trial over which Roberts has presided. It provides the potential for a confrontation with a president who has already antagonized him, even as the chief justice has tried to distance himself and the court from partisan politics.

"I think [the chief justice] is going to be very uncomfortable with it," said Carter Phillips, a longtime Washington lawyer who is among the most prolific practitioners before the Supreme Court. "He will look the part and will act the part, but I'm sure he'd rather not have the part."

History provides only two examples of a chief justice presiding over impeachment: Salmon Chase oversaw the impeachment trial of President Andrew Johnson in 1868, and William Rehnquist was in charge of President Bill Clinton's impeachment proceedings in 1999. Neither president was convicted, which requires a two-thirds vote in the Senate.

The only guidance the Constitution gives is: "When the President of the United States is tried, the Chief Justice shall preside." That is to avoid a potential conflict of interest because the vice president, who usually presides over the Senate, would theoretically benefit from a president's removal from office, according to Don Ritchie, the longtime Senate historian.

"That also gives a certain air of solemnity to the occasion," Ritchie said.

Since there are no cameras in the courtroom and like many justices he is somewhat averse to television interviews, Roberts is usually glimpsed by the public in a muted role at the State of the Union address or reading the oath of office to a newly elected president every four years.

Many scholars who have studied the impeachment process view the chief justice's role at an impeachment trial as similarly ceremonial. But on Tuesday, McConnell played up the power that Roberts has - noting that he would rule on any motions made by the opposing sides.

"The chief justice is in the chair. And senators are not allowed to speak," McConnell said. "So, that's the way it will be handled in the Senate."

That power is available should Roberts choose to use it.

As in a standard trial, both sides - the impeachment managers and Trump's defense counsel - would make their requests to Roberts. He then would have the discretion to decide motions - whether to admit evidence, dismiss the articles of impeachment or other incidental matters - on his own, according to two senior Senate Republican aides who have studied the trial process, although he could kick those questions to senators for a vote.

Any senator could ask to appeal a ruling by Roberts, and it would require only a majority vote to do so. During Clinton's 1999 trial, most decisions about witnesses and evidence - including whether former White House intern Monica Lewinsky would testify in person - were worked out by lawmakers.

Rehnquist, who had written a book about impeachment that focused in part on Johnson's trial, knew the role was limited. Chase had a more expansive role in the 1868 proceedings against Johnson, according to Ritchie, the former Senate historian.

But much of the precedents that guide the day-to-day proceedings changed dramatically earlier in the 20th century. Rehnquist adjusted, Ritchie said.

"And as a result, he got a great deal of respect from senators of both parties of being a fair presiding officer in that trial," he said. "I think it was not exactly the role he had anticipated playing, but he did play it, and he played it very well."

Roberts was a Supreme Court clerk for Rehnquist before the latter was elevated to chief justice in 1986, and Roberts was chosen by President George W. Bush to become chief justice himself after Rehnquist's death in 2005.

It was at his confirmation hearing that Roberts made his famous comments about calling balls and strikes.

"Judges are like umpires," Roberts said at the time. "Umpires don't make the rules. They apply them. The role of an umpire and a judge is critical. They make sure everybody plays by the rules. But it is a limited role. Nobody ever went to a ballgame to see the umpire."

"I think Rehnquist figured out he wanted to leave as small a footprint as possible," said Michael Gerhardt, an impeachment expert at the University of North Carolina's law school. He expects the same of Roberts.

"He'll come to this with everyone eager to learn what an umpire does," Gerhardt said.

Although the "Roberts Court" commenced when he became chief justice 14 years ago, a recent poll on the Supreme Court conducted by Marquette University's law school found that 66 percent of respondents said they didn't know enough about Roberts to venture an opinion on how he does his job.

The president and members of Congress have not been reluctant to judge. Trump has called him an "absolute disaster" and disappointment to Republicans because of what Trump said was Roberts's "bull----" vote in 2012 to uphold the major underpinnings of the Affordable Care Act.

The two squared off last Thanksgiving eve over Trump's criticism of an "Obama judge" who ruled against the administration.

Roberts issued a rare rebuke.

"We do not have Obama judges or Trump judges, Bush judges or Clinton judges," he said. "What we have is an extraordinary group of dedicated judges doing their level best to do equal right to those appearing before them. That independent judiciary is something we should all be thankful for."

Trump had the last word. "Sorry Chief Justice John Roberts, but you do indeed have 'Obama judges,' and they have a much different point of view than the people who are charged with the safety of our country," Trump tweeted.

Still, the president at times has acted as if the conservative court was an ally. "I DID NOTHING WRONG," Trump tweeted last April. "If the partisan Dems ever tried to Impeach, I would first head to the U.S. Supreme Court."

The court's precedents make it clear that impeachment is the business of Congress, not the courts. And when he presided over the Clinton impeachment, Rehnquist emphasized the complete role the Senate played in determining the rules of the hearing.

One of the few objections Rehnquist ruled on was a senator's concern about House members referring to senators as the equivalent of jurors. The objection, Rehnquist said, is "well taken, that the Senate is not simply a jury; it is a court in this case."

Rehnquist did try to exert his authority in other ways. Former senator Trent Lott, R-Miss., who was majority leader during the Clinton trial, recalled that on one of the trial days, he had just wanted to send his senators home after a long day.

The chief justice had other ideas.

"He said 'Wait, let's continue,' " Lott said in a recent interview.

Rehnquist "did not to my recollection play a controlling role or attempt to control" the proceedings, said Robert Bauer, who at the time was counsel to the Senate Democratic Caucus and now is a law professor at New York University.

Bauer said he would be surprised if Roberts's approach were any different from Rehnquist's, calling the chief justice an "institutionalist" whose primary concern is that the Supreme Court appear nonpartisan and neutral.

Still, the role appears to be an inherently awkward one for Roberts.

"From what I've read so far, the chief justice has an enormous amount of discretion. I'm sure John Roberts can't wait," said Sen. John Neely Kennedy, R-La. - then clarified that he was, indeed, speaking facetiously.

This article was written by Robert Barnes and Seung Min Kim, reporters for The Washington Post.