“Roberts, like many chief justices before him, has felt a duty to safeguard the institution’s reputation,” says Joan Biskupic, author of “The Chief,” a new Roberts biography, in an email. “The chief justice is leading a Court increasingly in his own image,” she adds. “He is positioned at the center in every way, and the law will likely be what he says it is.”

In this regard, Chief Justice Roberts now finds himself in a nearly unprecedented position: There is arguably only one other chief justice who has also been at the ideological center of his court. Amid a national climate of escalating partisanship, and the constitutional stress tests brought by the Trump presidency, he is likely to become one of the most closely-watched individuals in the country.

When Justice Brett Kavanaugh replaced the retired Justice Anthony Kennedy this term, Chief Justice Roberts became the court’s new ideological center, or “swing vote.” And, observers say, there are some signs that the conservative chief justice is becoming more willing to join his more liberal colleagues than in the past.

There is a convention that U.S. Supreme Court history is described not in “eras” or “ages,” or even decades, but by chief justices. Think “the Marshall Court” and “the Brennan Court.”

Chief Justice John Roberts has never understood that convention. The chief doesn’t have that much more power than the other justices, he has said. Nevertheless, “the Roberts Court” has been the shorthand since his confirmation in 2005.

Years from now, 2019 may be seen as the court term when that moniker starts to reflect the reality.

When Justice Brett Kavanaugh replaced the retired Justice Anthony Kennedy this term, Chief Justice Roberts became the court’s new ideological center, or “swing vote” when justices are deadlocked. And, observers say, there are some signs that the conservative chief justice is becoming more willing to join his more liberal colleagues than in the past. That said, this Supreme Court is the most conservative in decades.

Tuesday provided another data point, with the justices hearing oral arguments in two cases regarding partisan gerrymandering – one of several issues where Justice Kennedy was considered a pivotal vote.

In this regard, Chief Justice Roberts now finds himself in a nearly unprecedented position. He is only the 17th chief justice in U.S. history, and there is arguably only one other who has also been at the ideological center of his court. Amid a national climate of escalating partisanship, and the constitutional stress tests brought by the Trump presidency, he is likely to become one of the most closely-watched individuals in the country.

Chief Justice Roberts, raised a Roman Catholic, has been consistently conservative on social issues, including same-sex marriage and LGBTQ rights, as well as on voting rights, gun control, and campaign finance. But he has oscillated in a few recent cases, and appears more mindful of the court’s institutional role in American democracy.

“Roberts, like many chief justices before him, has felt a duty to safeguard the institution’s reputation,” says Joan Biskupic, author of “The Chief,” a new Roberts biography, in an email. “The chief justice is leading a Court increasingly in his own image,” she adds. “He is positioned at the center in every way, and the law will likely be what he says it is.”

Guarding institutional integrity

In practice, the chief justice only has a few extra powers that associate justices do not. The chief moderates conferences, where the justices discuss cases, and he assigns the opinion when he is in the majority.

More importantly, perhaps, the chief justice guards the Supreme Court’s institutional integrity. “The court has this position institutionally where the only power it has is people’s voluntary compliance with its decisions,” says Sara Benesh, a political scientist at the University of Wisconsin-Milwaukee. The court needs to appear above politics whenever possible, she adds, and “the chief is in a particularly important position on that.”

Indeed, Chief Justice Roberts’ public comments this term have emphasized the apolitical aspirations of the judiciary. “People need to know that we’re not doing politics.... We’re applying the law,” he said in Nashville last month. “We do not have Obama judges or Trump judges, Bush judges or Clinton judges,” he said in response to criticism from President Donald Trump last November about a migrant asylum ruling.

Ms. Biskupic says that when it comes to safeguarding the reputation of the court “his mission has become more complicated in our polarized times.”

“He had to think long and hard about when to go public with concerns about President Trump’s complaints regarding the judiciary,” she adds.

More broadly, Chief Justice Roberts has a long-held dislike for what he has called the “personalization” of politics and law. The court should function as a single voice, not nine individual interpretations of the law, he told Jeffrey Rosen of the Atlantic in 2007. “I think it’s bad, long term, if people identify the rule of law with how individual justices vote,” he told Mr. Rosen.

The Marshall Model

That belief is a reason why, soon after Justice Roberts became chief, he took former Chief Justice John Marshall as a role model.

The architect of unanimous opinions, including two that shaped the high court’s power of judicial review and the scope of Congress’ power over the states, Marshall “established the institutional prestige and power of the court,” says Timothy Huebner, a history professor at Rhodes College in Memphis, Tennessee.

But Chief Justice Roberts told Rosen that over the past few decades the Supreme Court “has been eroding, to some extent, the capital that Marshall built up.”

Chief Justice Roberts has acknowledged that Marshall worked in a different age. The law was relatively undeveloped, and the court had to unify if it wanted any institutional power at all. Legal philosophies have hardened now – including the chief justice’s – and as Congress has lost institutional strength to partisan gridlock, the country has turned to the high court to guide American democracy.

“On [polarizing] issues the court is likely to be split like the country is, and in those circumstances an institutionalist is only going to get so far,” says Professor Huebner. That has forced Chief Justice Roberts “to really cut his own path.”

The chief justice has been prolific at forging unanimous decisions – nine of the court’s first 10 decisions this term were unanimous, the third time that has happened since he became chief – but often only by deciding cases on narrower grounds.

When a case has hit on firm ideologies, unanimity has been elusive. Up to this term, the chief justice himself has rarely joined his liberal colleagues in a 5-4 decision. One of those exceptions was his support of the Affordable Care Act in 2012, which earned the wrath of conservatives.

In her book, for which she interviewed the chief justice (in eight off-the-record interviews) and many of his colleagues, Ms. Biskupic writes that the chief justice is often torn between guarding the court’s institutional integrity and carrying out a conservative agenda.

“He has had considerable influence on his colleagues. Sometimes, however, he runs into the firm ideologies of his colleagues,” she says. “Sometimes he himself is fixed in his ideology and refuses to compromise.”

More oscillation ahead?

There has been only one other chief justice who was also a swing justice, historians say: Charles E. Hughes, a moderate progressive chief in the 1930s who voted with conservative colleagues to strike down New Deal laws before shifting to join liberal colleagues in upholding other New Deal laws.

Most academics have described Hughes as being his court’s ideological center, wrote William Leuchtenburg in a 2005 law review article. “But [he never] had a cohesive centrist position. Rather, [he] oscillated from one side to the other.”

That is perhaps what can be expected of Chief Justice Roberts in the coming years, some court-watchers say. And there has been some recent evidence to suggest the chief justice may be oscillating more than in the past.

In December, he sided with the court’s four liberals in refusing to lift a lower court stay on a policy that would have denied asylum to those who illegally cross the U.S.-Mexico border. In February, he joined them again to stop a Louisiana abortion law from going into effect, and they voted together later that month to order a state court to look again at the mental competency of a death row inmate who can’t remember his crime.

“While Roberts is by no means moving into the Court’s liberal camp, the data concerning his ideological voting behavior does intimate a mild liberalizing over time,” wrote Adam Feldman in a recent analysis for the Empirical SCOTUS blog.

Some say it’s too early to know if he will change how he decides issues when unanimity isn’t possible. Based on his record, today’s oral arguments on partisan gerrymandering in North Carolina and Maryland are unlikely to provide new hints.

While Justice Kennedy had expressed some openness to the high court placing limits on the inherently partisan redistricting process, Chief Justice Roberts could barely hide his alarm at the thought. “We will have to decide in every [partisan gerrymandering] case whether the Democrats win or the Republicans win,” he said during oral argument in a similar case last term. “That is going to cause very serious harm to the status and integrity of the decisions of this court in the eyes of the country.”