WASHINGTON (Reuters) - U.S. Chief Justice John Roberts is poised to serve a highly visible though largely ceremonial role in the Senate impeachment trial of President Donald Trump due next month. But it is in his day job on the Supreme Court that the mild-mannered jurist could have a bigger impact on Trump’s presidency.

FILE PHOTO: Chief Justice of the United States John G. Roberts is seen during a group portrait session for the new full court at the Supreme Court in Washington, U.S., November 30, 2018. REUTERS/Jim Young/File Photo

Roberts, 64, is set to preside over the trial in which the 100 U.S. senators will serve as jurors to decide whether to convict the president and remove him from office, an unlikely result considering Trump’s fellow Republicans control the chamber and a two-thirds majority is needed to oust him.

While the senators - not Roberts - set the rules for the trial and determine its outcome, he is positioned to play a central role in deciding significant cases now before the nine-member court that will directly impact Trump.

It is in the marble-lined corridors of the Supreme Court across the street from the U.S. Capitol, hidden from the television cameras, where Roberts wields real power.

The justices will hear arguments and rule by the end of June - in the heat of the 2020 presidential race - on Trump’s bid to keep details of his finances secret after lower courts ordered that his accounting firm and two banks turn over records to congressional investigators and a New York City prosecutor.

Roberts is considered the ideological center of a court with a 5-4 conservative majority. As such, he could very well represent the decisive vote. Roberts is known for his cautious approach to major cases, sometimes disappointing fellow conservatives.

The Democratic-led House of Representatives on Wednesday made Trump only the third U.S. president to be impeached, passing two formal charges - abuse of power and obstruction of Congress - arising from his request that Ukraine investigate political rival Joe Biden.

The court’s rulings in the financial records cases could set precedents consequential not only for Trump but for other presidents for decades to come.

With Democratic House lawmakers and a Democratic prosecutor in New York issuing subpoenas for his financial records, the businessman-turned-politician has argued for broad presidential immunity. Rulings in Trump’s favor could handcuff Congress and prosecutors in investigating any sitting president. Unlike other recent presidents, Trump has refused to disclose his tax returns.

AN UNCOMFORTABLE EXPERIENCE

The impeachment trial promises to be uncomfortable for Roberts, who prefers to fly under the radar even while he has steered the court in a rightward direction since being appointed as chief justice in 2005 by Republican President George W. Bush.

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“My sense is that the chief doesn’t want to make himself the story,” said Sarah Binder, a scholar at the Brookings Institution think tank.

Those who know Roberts, including former law clerks, have said he will take his trial role seriously and, as a history buff, is likely reading up on the previous impeachment trials of Presidents Andrew Johnson and Bill Clinton, both of whom were acquitted.

His job is to keep the trial on track, though Roberts could be called upon to rule on whether certain witnesses should appear. Senators could reverse him if a majority disagrees with any ruling he makes.

In Clinton’s 1999 impeachment trial, then-Chief Justice William Rehnquist had “relatively little to do,” said Neil Richards, a professor at Washington University School of Law in St. Louis who was one of Rehnquist’s law clerks at the time.

“I think Chief Justice Roberts is likely to approach his role ... the way he has approached his judicial career to date: doing his best to be impartial, doing his best to preserve the dignity of his judicial office,” Richards added.

Roberts has not spoken publicly about his impending role. During a rare public appearance in New York in September, he appeared concerned about Washington’s hyperpartisan politics.

“When you live in a polarized political environment, people tend to see everything in those terms. That’s not how we at the court function,” Roberts said.

Roberts has the reputation as a traditional conservative and strong defender of the Supreme Court’s institutional independence. Roberts, raised in Indiana and educated at Harvard Law School, served in Republican President Ronald Reagan’s administration in the 1980s and was appointed first to a federal appeals court and later to the Supreme Court by Bush.

Roberts is often viewed as an incrementalist in his judicial philosophy, mindful that the Supreme Court risks its legitimacy if its conservative majority is seen as moving too aggressively to the right in its rulings.

He has voted consistently with his fellow conservatives -against gay and abortion rights and for expanded religious liberty and gun rights. But in 2012, Roberts sided with the court’s liberal bloc and cast the deciding vote to uphold the healthcare law dubbed Obamacare, signed by Democratic President Barack Obama in 2010 and reviled by conservatives.

In June, Roberts joined the liberal justices and cast the deciding vote in a 5-4 ruling blocking Trump from adding a citizenship question to the 2020 census that critics said was intended to deter immigrants from taking part in the decennial population count.

Roberts publicly differed with Trump in November 2018, taking the rare step of issuing a statement defending the federal judiciary after the president lashed out at judges who had ruled against him.

An independent judiciary, Roberts said, “is something we should all be thankful for.”