Many scholars have said that Chief Justice Roberts sought to balance his own conservatism with his desire to build faith in the law and the nation’s legal institutions. But it was still striking to hear Mr. Roberts, who arrived on the court in 2005 appointed by George W. Bush, announce the upholding of the central legislative pillar of the Obama administration. He did arrive on the bench asserting the desire to restore the court’s reputation and reduce partisan rhetoric. But he was seen by many, at least on the left, as a right-winger more devoted to conservative politics than the purity of the law. That could change.

“This could be a huge day in the evolution of Chief Justice Roberts as a great chief justice,” Laurence H. Tribe, the liberal Harvard law professor, said. Mr. Tribe, who taught Mr. Roberts, said he had not opposed his nomination because he believed Mr. Roberts was less of an ideologue than many charged. “I have some sense of gratification,” he said.