With an oath of impartiality, Chief Justice John G. Roberts Jr. on Thursday became only the third American sworn to preside over a presidential impeachment trial.

How he fulfills that pledge will have obvious consequences for President Trump. But it also will shape the public image of the nation’s 17th chief justice, and it holds ramifications for the Supreme Court and federal judiciary he leads. He portrays both as places where partisan politics have no purchase.

“And now he crosses First Street, where it’s all about partisan politics,” said Harvard law professor Richard Lazarus, referring to the roadway in Washington that separates the Supreme Court from Congress.

There are obvious risks for Roberts, but Lazarus said he doesn’t believe the chief justice will be particularly “risk-averse.”

“I don’t think he’s going to look like a potted plant,” said Lazarus, who has known Roberts since law school and has taught summer courses with him after he became chief justice.

“He’s not going to erode the stature of the chief justice and the Supreme Court in the process by looking like an insignificant person.”

The Constitution dictates that the chief justice preside in a presidential impeachment trial, but it gives no guidance beyond that.

Under Senate rules, Roberts swears in senators as jurors, and all questions from senators are submitted in writing to him, and then directed to the House impeachment managers, the president’s legal team or witnesses. The rules also say the chief justice “may rule on all questions of evidence.”

But unlike in a criminal court trial, with standards of proof required to find a defendant guilty, the question of whether to convict in an impeachment proceeding has historically been a political choice for individual senators who will cast their votes.

“John Roberts can, in a sense, do what he wants,” said Brenda Wineapple, author of “The Impeachers: The Trial of Andrew Johnson and the Dream of a Just Nation,” which details the 17th president’s impeachment proceedings in 1868.

Roberts’s predecessors have not left much of a road map. Chief Justice Salmon P. Chase did not shy from making procedural rulings in Johnson’s impeachment trial and was sometimes overruled by the Senate majority.

Chief Justice William H. Rehnquist took a far more hands-off approach during the 1999 impeachment trial of President Bill Clinton. His most memorable intrusion into the proceedings was to remind senators that they were not just jurors in the case, but the court as well, and thus responsible for setting the rules.

Roberts will make his own way, but Wineapple, like most other experts, believes he will hew more closely to the Rehnquist approach.

Roberts, who was a law clerk for Rehnquist in 1980, is “always thinking about the long view of history, what came before and what comes after,” Wineapple said. “In that sense I’d imagine he’s going to be extremely cautious about being perceived as listing one way or another because that flies in the face of the myth that the court is above partisan politics.”

In his year-end report on the judiciary, Roberts wrote what human resources departments might describe as his goal for 2020: “As the New Year begins, and we turn to the tasks before us, we should each resolve to do our best to maintain the public’s trust that we are faithfully discharging our solemn obligation to equal justice under law.”

Some argue that cannot be accomplished in the impeachment trial simply by being passive.

“He’s going to want to be somewhat hands-off, but the fact that the American people want something that looks like a fair trial means he’s going to need to be the judge,” said Elizabeth Wydra, president of the liberal Constitutional Accountability Center. “Being the judge means issuing rulings on key aspects of a fair trial, including whether or not you have documents and witnesses.”

In a column she wrote on the subject this week, Wydra said, “Roberts may simply be unable to avoid stepping into the fray and getting his robe dirty.”

Paul M. Collins Jr., a University of Massachusetts at Amherst political scientist who studies the judiciary, described Roberts’s dilemma.

“Any decision he makes in the president’s favor will be interpreted as partisan since Roberts is a Republican,” Collins said. “Yet any decision he makes against the president will be interpreted as biased, too.”

That is because of the history of the two men. During the 2016 campaign, Trump declared Roberts a “disaster” and his nomination a mistake because of his vote that saved the Affordable Care Act.

Roberts in 2018 issued a rare rebuke of Trump’s criticism of federal judges as biased against him, and he frequently lauds the independence of the federal judiciary.

Senate Majority Leader Mitch McConnell (R-Ky.) has alternately emphasized and minimized the chief justice’s role in the impeachment trial. Lately, as Democratic demands for witnesses and documents have grown in the Senate, McConnell has reminded that a majority of senators will make the rules.

“I would anticipate the chief justice would not actually make any rulings,” McConnell said recently. “He would simply submit a motion to the body and we would vote.”

Among those Roberts is turning to for advice in preparation for his new role is his counselor, Jeffrey P. Minear, and James Duff, who was Rehnquist’s top aide during Clinton’s trial. Roberts subsequently appointed Duff in 2015 to oversee the federal judiciary system as head of the Administrative Office of the U.S. Courts.

He is also likely to rely heavily on Senate parliamentarian Elizabeth MacDonough for her expertise in the ways of the Senate.

Roberts was famous for his preparation when he was in private practice and arguing cases at the Supreme Court. He anticipated hundreds of possible questions from the justices, and Lazarus imagines the same sort of process is at work now.

“Compared to now, there was great partisan agreement during the Clinton trial,” Lazarus said, referring to unanimous agreements between Republicans and Democrats about how to conduct the proceedings.

“The chief has to preside over a Senate where there is no agreement between the majority and the minority, and he has to prepare for not knowing what McConnell or Schumer might do,” said Lazarus, referring to Senate Minority Leader Charles E. Schumer (D-N.Y.), who has pressed his Republican counterpart, unsuccessfully, to insist on calling on witnesses during the trial.

But Roberts also will be judged in the greater context of the Supreme Court, which will continue its work as the impeachment trial unfolds. The court has a controversial docket that threatens to make plain its conservative-liberal divide on issues such as abortion, gun control and legal protections for LGBT workers.

The justices currently are considering whether the Trump administration skirted the law when trying to end the Obama-era program protecting young immigrant “dreamers” from deportation. They have accepted cases in which Trump has sued to block release of his financial records to prosecutors and congressional committees.

And they’ve been asked to fast-track efforts that once again challenge the legality of the Affordable Care Act, President Barack Obama’s landmark health-care legislation, the elimination of which has been a priority for Trump and other conservatives.

Almost surely headed to the high court is the issue of whether the judiciary has a role to play when the executive branch defies congressional subpoena.

Such cases are another reason for Roberts to be cautious in his role overseeing the impeachment trial, said former senator Trent Lott, who served as majority leader during Clinton’s impeachment.

For instance, Lott said, Trump might try to block testimony at the Senate trial from current or former administration officials through a claim of executive privilege.

“You don’t want to wade into that. He might end up having to rule in the Supreme Court,” the Mississippi Republican said of the chief justice.

Lott recalled that at the conclusion of Clinton’s five-week proceedings, senators were so appreciative of Rehnquist’s low-profile performance that they rose to give him a standing ovation and presented him with a “Golden Gavel,” awarded by Senate tradition to those who have presided over the body for 100 hours.

“I am not sure it quite reached 100 hours, but it is close enough,” Lott said at the time.

Rehnquist responded: “It seemed like it.”

Correction: A previous version of this story misidentified the Senate parliamentarian. She is Elizabeth MacDonough.