With a Senate trial in the impeachment of US President Donald Trump set to begin in earnest next week, John Roberts, the chief justice of the Supreme Court, has been sworn in to preside over the upcoming proceedings - a role only two of his predecessors have ever filled.

This position has the potential to be largely ceremonial, offering the judicial gravitas of a sitting justice from the highest court in the land to what will largely be a mostly political arena. However, Roberts will also likely have the ability to influence some of the key questions surrounding the rules that will define the upcoming trial - in particular, what evidence will be allowed and the hotly contested issue of whether new witnesses will be permitted to testify and who those witnesses will be, analysts say.

But how Roberts, who was appointed to the prestigious position by Republican President George W Bush and who has since become the sole swing vote on the Supreme Court, will approach the position remains to be seen.

More:

Many point to Roberts's recent comments on the importance of maintaining the apolitical nature of the Supreme Court, which is the highest court in the US and resolves the constitutionality of lower court rulings, as an indication of how he will fulfil his unique task.

Justice's serve for life. While they can be impeached, they cannot be fired. These stipulations are meant to elevate the court above partisan politics.

The Constitution requires that a chief judge "shall preside" over presidential impeachment trials in the Senate. It does not offer further details on the chief justice's role - leaving open to interpretation how much chief justices can affect a trial. Such questions can be decided by a Senate vote. Beyond those agreed parameters, the degree of involvement, in the past, has largely been left to the discretion of the individual chief justice.

"Impeachment of a US president is an unusual circumstance," Thomas M Keck, a professor of political science at Syracuse University told Al Jazeera. "(The chief justice) is there to oversee a trial, which is something that should be well within his comfort zone. But it's a trial conducted by elected partisan officials. It's not a court, the US Senate."

Senate Majority leader Mitch McConnell has not yet produced the rules for the upcoming trial.

However, if the rules of the 1999 Senate impeachment trial of former President Bill Clinton are any indication, Roberts role will likely allow him to rule on procedural motions, although any rulings can then be overruled by a simple majority. Republicans currently hold 53 seats in the 100-member chamber. He can also send motions directly to a Senate vote without a ruling, under the past rules.

Who is Chief Justice John Roberts?

Roberts was born in Buffalo, New York but spent most of his formative years in Indiana. He attended Harvard University as an undergraduate and then as a law student, becoming the managing editor of the Harvard Law Review.

He later served as a law clerk to Supreme Court Justice William Rehnquist, one of only two other chief justices to preside over an impeachment trial.

Roberts also served in prosecutorial and advocate roles for former Presidents Ronald Reagan and President George HW Bush.

In 2003, Roberts was confirmed as a federal judge in the DC Circuit Court of Appeals, after being appointed by President George W Bush. Two years later, Bush nominated him to fill the seat of retiring Justice Sandra Day O'Connor. But when Rehnquist died, Bush instead appointed Roberts to the role of chief justice.

"His whole adult professional life led the Bush administration to have high confidence that he would be a conservative supreme court justice and he has been," said Keck.

When Justice Anthony Kennedy retired in 2018, Roberts became the sole swing vote on a court composed of five Republican appointees and four Democrat appointees.

"If you want to map him ideologically on the courts, he is a conservative, centre-right (justice)," Adam White, a law professor at George Mason University and a Resident Scholar at the American Enterprise Institute, told Al Jazeera.

"On the current Supreme Court, he sits dead set in the middle of the nine justices," he said.

Despite being a conservative, Roberts's votes in rulings that upheld key components of former President Barack Obama's signature healthcare policy did not win him many fans among Republican politicians, White added.

In some other key rulings, including a deciding vote striking down a Trump administration move to add a citizenship question on the US census, Roberts has shown a willingness to break from the Republican political agenda, said Keck, who added that Roberts's voting history generally remains "very conservative".

Roberts has also largely remained out of the national spotlight. However in 2018, in a rare rebuke to a sitting president from a chief justice, Roberts responded to Trump's claim that an "an Obama judge" had struck down an administration policy that aimed to deny asylum to people who did not enter the US at an official port of entry.

"We do not have Obama judges or Trump judges, Bush judges or Clinton judges," Roberts said in a statement at the time. "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."

What powers does the chief justice have in the trial?

If the Senate trial rules resemble those used during the Clinton impeachment, Roberts will have the ability to rule "on any kind of procedural question that comes up: can we introduce this document? Is it materially relevant or not? Can we call this witness? Can we have some extra time?" said Keck.

Even though a ruling can be overturned by a simple majority Senate vote, it could still prove influential in the proceedings, he added.

{articleGUID}

"The fact that the chief justice rules on it first is at least a bit of an extra hurdle, if you're a moderate Republican senator running for reelection," said Keck.

However, given the format, "the ultimate decision-makers are whatever moderate Republican senators might or might not be willing to join with Democrats on some of these procedural questions," he said.

How will Chief Justice Roberts approach the role?

The two previous impeachment proceedings, Andrew Johnson's post-US Civil War 1868 trial and Clinton's 1999 trial, saw very different approaches from the presiding chief justices, said Alan Baron, a former special counsel to the House of Representative during four impeachment inquiries against federal judges.

Chief Justice Salmon Chase, who presided over Johnson's trial, "saw himself as playing a very central role, and from his point of view, this is like a trial, he's a trial judge, and he made all kind of rulings".

Meanwhile, Rehnquist was famously hands-off.

"He wasn't a potted plant, but he wasn't exactly a real player," said Baron.

Roberts, in the few public statements he has made in recent times, including his 2019 year-end report [PDF] on the federal judiciary, has called for maintaining the apolitical integrity of the Supreme Court.

"He thinks a lot about the court's place in American politics and democracy, and he's given a number of speeches over the years warning people not to think of the court as a political body, not to politicise the court, not to expect the court to settle every political argument," said White.

Experts generally agree that Roberts will try to maintain that neutrality going into the impeachment proceedings. But how such neutrality will be executed is very much up for debate.

Keck argues that in dealing with an administration that has "posed a test for many norms and institutions of constitutional democracy" and a Republican Senate leadership that has said they will work closely with the White House defence, a light hand from Roberts may not come across as fair.

"Being a neutral even-handed judge could theoretically require an effort to put some constraints on the partisan majority," said Keck.

But having as small a role as possible is likely the best way to avoid the appearance of politicisation, argues White.

"If he were to sort of be extra restrained and not rule on things, would he be criticised? He'll get some criticism from those who want to see him rule on things," said White. "But I think that to really energetically become a participant in the proceedings, that would politicise things much, much more."

"It's hard to accuse him of politicisation when what he's doing is restraining himself, not inserting himself."