John Roberts, Trump Tormentor

But he is also an institutionalist, legal scholars say, one who’s worried about the court’s independence and credibility in a deeply polarized political moment.

“The court’s role as an institution in American government has come under increased pressure,” said Leah Litman, a constitutional law professor at the University of Michigan who clerked for Justice Anthony Kennedy. “If it looks very political and partisan, it might not be long for this world in its current form.”

While public confidence in the court remains high, the bitter political fight over Brett Kavanaugh’s nomination dealt a blow to how some voters view the third branch of American government.

After Justice Kavanaugh’s confirmation, Chief Justice Roberts, who chooses his public words carefully, felt compelled to address how partisan dysfunction in Congress had transformed the judiciary into a kind of political spoils system.

“We do not sit on opposite sides of an aisle,” he said in October 2018 in remarks at the University of Minnesota. “We do not caucus in separate rooms. We do not serve one party or one interest. We serve one nation. And I want to assure all of you that we will continue to do that to the best of our abilities whether times are calm or contentious.”

Liberals and some legal scholars say Mr. Roberts is responding, at least in part, to an emboldened conservative legal movement that is eager to push the bounds of legal precedent.

They point to the Trump tax case, in which the president’s lawyer argued that Mr. Trump had immunity beyond what past presidents had asserted, as an example. At one point, his lawyers even argued that Mr. Trump could, as he once famously mused, shoot someone on Fifth Avenue and be protected from investigation.