Chief Justice John Roberts has tried to prevent the Supreme Court from being seen as just another political body, but when he presides over President Trump Donald John TrumpHR McMaster says president's policy to withdraw troops from Afghanistan is 'unwise' Cast of 'Parks and Rec' reunite for virtual town hall to address Wisconsin voters Biden says Trump should step down over coronavirus response MORE’s likely impeachment trial in the Senate, the partisan glare will be hard for him to avoid.

The 64-year-old chief justice who famously said judges should simply call balls and strikes will now hold influence over the most bitterly partisan impeachment trial in modern American history, a situation more akin to umpiring a bench-clearing brawl.

The contentious affair threatens to put Roberts, a George W. Bush appointee, under political pressure from all sides, a role court watchers say the mild-mannered jurist will assume with great reluctance.

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

The timing of Trump’s impeachment trial could hardly be less auspicious for a chief justice eager to avoid the political fray, especially as Roberts has morphed into something of a swing vote between the court’s reliably conservative and liberal blocs.

The Senate trial, expected to start in January, will also unfold as the Supreme Court prepares to hear arguments in the coming months in a landmark separation of powers fight involving efforts by House Democrats and New York state prosecutors to obtain years of Trump’s financial records and tax returns.

Roberts could cast the deciding vote to disclose or shield Trump’s financial records and may tip the scale in other hot-button cases that involve LGBT and abortion rights and the deportation status of nearly 700,000 young undocumented immigrants.

Decisions in these politically charged disputes should arrive this summer, just months before Americans head to the polls to determine if Trump gets a second term in the White House.

Trump and Roberts have at times had a difficult relationship.

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The chief justice pushed back in a speech after the president branded a judge on the 9th Circuit Court of Appeals "an Obama judge" for a 2018 decision that Trump disagreed with.

"We do not have Obama judges or Trump judges, Bush judges or Clinton judges," Roberts responded at the time, defending the federal judiciary.

Trump would later say that he held "a lot of respect" for Roberts, even as the president doubled down on his criticism of the appeals court decision.

The political drama surrounding Roberts as he prepares to preside over the impeachment trial is likely to heighten his sensitivity to the public perception of the high court and the judiciary.

“One of his goals will be to preserve the image of the Supreme Court as a kind of neutral arbiter so that the legitimacy of his institution is protected,” said Ian Ostrander, a political science professor at Michigan State University. “Stepping into a partisan battle within a different branch is not ideal.”

Trump is expected to be acquitted by the Republican-controlled Senate, where a two-thirds vote is required for a conviction. But there are still many unanswered questions about the trial and its rules and how Roberts chooses to carry out his constitutionally prescribed duties as the trial's presiding officer. Legal experts say he is likely spending time studying long-standing Senate rules and past impeachment proceedings.

But when the trial begins, Roberts will have to make those 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.

House Democrats this month took the historic step of impeaching Trump. But they delayed sending the two House-passed articles of impeachment to the Senate as talks broke down between the top two Senate leaders over the terms of Trump’s trial.

The Constitution states that the U.S. House of Representatives "shall have the sole Power of Impeachment" and the Senate "shall have the sole Power to try all Impeachments" and when the president is on trial, "the Chief Justice shall preside." But the Constitution is silent about how a presidential impeachment trial should work in practice.

The starting point is a nine-page set of rules the Senate adopted in the mid-1980s. These Senate trial rules make clear that senators themselves have ultimate authority over all critical aspects of the proceeding.

The upper chamber may also adapt the 1986 trial rules through supplemental agreements, which happened both before and during President Clinton's impeachment trial.

In Trump’s case, however, negotiations between Senate Majority Leader Mitch McConnell Addison (Mitch) Mitchell McConnellMcConnell focuses on confirming judicial nominees with COVID-19 talks stalled McConnell accuses Democrats of sowing division by 'downplaying progress' on election security Warren, Schumer introduce plan for next president to cancel ,000 in student debt MORE (R-Ky.) and Senate Minority Leader Charles Schumer Chuck SchumerMcConnell accuses Democrats of sowing division by 'downplaying progress' on election security Warren, Schumer introduce plan for next president to cancel ,000 in student debt Schumer lashes out at Trump over 'blue states' remark: 'What a disgrace' MORE (D-N.Y.) deadlocked 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.

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The default trial rules guide the somewhat complicated and technical power-sharing arrangement between the Senate, which in essence serves as both judge and jury, and the presiding officer. Most importantly, the rules also leave some room for the chamber and the presiding officer to decide among themselves the shape of the trial.

Legal experts say Roberts hopes to follow the example set by his late mentor Chief Justice William Rehnquist at Clinton's 1999 trial. Rehnquist, for whom Roberts clerked in the 1980s, once mused that during Clinton’s proceeding he "did nothing in particular, and did it very well," lifting a line from Gilbert and Sullivan.

Eric Claeys, a law professor at George Mason University and former Rehnquist clerk, said Rehnquist’s approach was framed by the 1986 rules.

The rules say the presiding officer “may rule” on all questions of evidence, like instances where the relevance and significance of a document or witness testimony is unclear. However, a single senator can appeal the ruling, triggering a Senate vote, where some say Roberts would break a 50-50 tie, though that is disputed.

The presiding officer also has the option to stay mum on an evidentiary question and send it directly to the Senate for an up-or-down vote.

Rehnquist generally avoided this option during Clinton’s 1999 impeachment trial, Claeys said. Instead, Rehnquist applied the relevant precedents, then left it up to senators to decide whether to reverse his decision.

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“I don’t think that Chief Justice Roberts will play a bigger role in President Trump’s impeachment trial than Chief Justice Rehnquist did in President Clinton’s,” Claeys said. “I expect Roberts will follow the same strategy.”

However, some legal experts believe today’s more intensely partisan atmosphere may force Roberts to depart from the course charted by his predecessor.

Rehnquist presided over a Senate whose leaders managed to work out compromises and cut deals on impeachment trial rules, but Roberts has no guarantee the toxic McConnell-Schumer relationship will yield the same result.

If no Senate deal emerges on the issue of witnesses, it could draw Roberts deeper into the partisan fray, some experts believe.

Robert Tsai, a constitutional expert and law professor at American University, says he expects Democrats to seek subpoenas for live testimony from Mulvaney and Bolton, and perhaps even Trump’s personal lawyer Rudy Giuliani Rudy GiulianiGrand jury adds additional counts against Giuliani associates Lev Parnas and and Igor Fruman Juan Williams: Breaking down the debates Giuliani criticizes NYC leadership: 'They're killing this city' MORE.

Those could be among the toughest rulings Roberts may be forced to make in the spotlight.

“Here is perhaps the most controversial issue Roberts might be asked to rule upon, and yet it’s also one of the most important to issues of basic fairness,” Tsai said. "The problem is that the politics, which are unavoidable, make things explosive whichever way he rules.”