Chief Justice John Roberts will soon discover firsthand that while the Supreme Court and the Senate sit on adjacent Washington city blocks, the two institutions occupy separate worlds.

Roberts on Thursday appeared in the Senate in his black robes to preside over President Trump Donald John TrumpObama calls on Senate not to fill Ginsburg's vacancy until after election Planned Parenthood: 'The fate of our rights' depends on Ginsburg replacement Progressive group to spend M in ad campaign on Supreme Court vacancy MORE's impeachment trial, leaving the collegiality of the court for a chamber marked in recent years by partisan fighting.

The chief justice was led by a procession of Judiciary Committee and Rules Committee members to the well of the chamber. There, he raised his right hand and swore to do "impartial justice" — the kind of oath he is more accustomed to hearing from advocates before the Supreme Court.

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Roberts’s opening act as presiding officer was uncontroversial and largely scripted, starting with the swearing in of the Senate’s lawmakers. But when the trial officially begins on Tuesday he will find himself in the midst of a highly charged political battle.

The Constitution gives the Senate “the sole Power to try all Impeachments” and tasks the Supreme Court chief justice with the role of presiding officer when the president is on trial. But the Constitution is silent about the trial’s actual mechanics, raising questions about whether the mild-mannered, 64-year old jurist will play a mostly ceremonial role overseeing the trial or be dragged into the political fight and forced to make consequential decisions.

Legal experts say Roberts hopes to follow the largely hands-off example set by his late mentor, Chief Justice William Rehnquist, during President Clinton's 1999 trial. Rehnquist once quipped that during the proceeding he "did nothing in particular and did it very well," leaving senators to plot their own course.

Court watchers paint Roberts as a reluctant participant in the impeachment drama. The chief justice has long worried about the federal bench being politicized and has sought to protect the high court's reputation from partisan sniping.

“He will look the part, and he will play the part, but he does not want the part,” Carter Phillips, a partner at Sidley Austin who has argued dozens of cases before the Supreme Court.

Whether Roberts can avoid controversy remains to be seen. Trump's impeachment trial promises to be a radical departure from the relatively calm grandeur of the high court. And today’s more intensely partisan atmosphere, compared to two decades ago, may force Roberts to stray from the noncontroversial course charted by Rehnquist.

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For Clinton's trial, senators unanimously passed a resolution that set the rules for the early phases of the proceeding, but the current Senate's leaders have been at an impasse for weeks over how to move proceed.

Before swearing in senators on Thursday, Roberts walked down the chamber’s brightly lit aisle between Senate Majority Leader Mitch McConnell Addison (Mitch) Mitchell McConnellObama calls on Senate not to fill Ginsburg's vacancy until after election Planned Parenthood: 'The fate of our rights' depends on Ginsburg replacement Progressive group to spend M in ad campaign on Supreme Court vacancy MORE (R-Ky.) and Senate Minority Leader Charles Schumer Chuck SchumerPelosi orders Capitol flags at half-staff to honor Ginsburg Ginsburg in statement before her death said she wished not to be replaced until next president is sworn in Democrats call for NRA Foundation to be prohibited from receiving donations from federal employees MORE (D-N.Y.). The two have been deadlocked since last month after Democrats requested that Roberts issue subpoenas to acting White House chief of staff Mick Mulvaney Mick MulvaneyMick Mulvaney to start hedge fund Fauci says positive White House task force reports don't always match what he hears on the ground Bottom line MORE, former national security adviser John Bolton John BoltonDiplomacy with China is good for America The Hill's Morning Report - Sponsored by The Air Line Pilots Association - Pence lauds Harris as 'experienced debater'; Trump, Biden diverge over debate prep DOJ launches probe into Bolton book for possible classified information disclosures MORE and two other officials.

If the two parties are unable to reach a deal on the issue of calling witnesses to testify, it could draw Roberts deeper into the fray.

Guiding the arrangement between the Senate and Roberts is a nine-page set of rules the chamber adopted in the mid-1980s, though this template is likely to be supplemented with additional agreements.

The Constitution makes clear that senators themselves have ultimate authority over all critical aspects of the proceeding, but the 1986 Senate rules gives the presiding officer an opening to help define the trial's contours.

The standing rules say the presiding officer “may rule” on all questions of evidence, such as instances where the relevance and significance of a document or witness testimony is unclear. However, a single senator can appeal the ruling, triggering a vote in the Senate. And any decision Roberts makes can be overturned by a simple majority vote in the Senate.

The presiding officer also has the option to sidestep an evidentiary question and send it directly to the Senate for an up-or-down vote. But Roberts, who famously said judges should simply call balls and strikes, may not be able to avoid controversial rulings so easily.

One open question in the Senate's impeachment trial procedure is whether the chief justice would be able to break a 50-50 tie, which could arise if three Senate Republicans were to break from their colleagues and vote with a unified Democratic caucus. Rulings on motions for witnesses or documentary evidence require only a simple majority.

The issue of a 50-50 split in a trial last arose in 1868, when Chief Justice Salmon Chase broke two ties during the impeachment trial of President Andrew Johnson.

Roberts will undoubtedly face pressure from both sides if he is confronted with such decisions, and Republicans have already begun laying out their views. A spokesman for McConnell told The Hill that any motion that saw a 50-50 tie would fail, a view shared by Sen. Roy Blunt Roy Dean BluntCDC tells Congress it urgently needs billion for vaccine distribution On The Money: Trump undercuts GOP, calls for bigger COVID-19 relief package | Communities of color hit hardest financially by COVID-19 | Businesses, states pass on Trump payroll tax deferral Trump undercuts GOP, calls for bigger COVID-19 relief package MORE (R-Mo.), the chairman of the Senate Rules Committee.

But others disagree. Harry Litman, a former U.S. attorney and a professor of constitutional law, argued in a Washington Post op-ed that Roberts can break ties.

Legal experts say Roberts has likely spent time studying long-standing Senate rules and past impeachment proceedings to prepare for his new role. But that historical knowledge may be cold comfort for a chief justice who is more accustomed to drawing conclusions on critical issues only after thorough deliberation and outside of public view.

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Once the trial begins, Roberts may have to make momentous decisions under the glare of cameras while being pressured by both parties and with the president likely watching and weighing in on the proceedings in real time.

Trump is expected to be acquitted by the Republican-controlled Senate, where a two-thirds vote is required for a conviction, but how that process plays out will have high stakes not just for Trump and senators but Roberts too. The chief justice, whose 2012 ruling to uphold ObamaCare infuriated conservatives, is no stranger to controversy. The impeachment trial will go far in determining the public perception of Roberts and by extension the reputation of the federal courts.

“He will, at all times possible, try to let the political players craft political compromises and limit his role only to that which is truly necessary,” national security lawyer Brad Moss predicted.

“Anyone who expects Chief Justice Roberts to willfully put himself into the political muck is kidding themselves.”

Jordain Carney contributed.