Chief Justice John G. Roberts Jr. surprised many on Thursday by providing the crucial fifth vote for upholding President Obama’s health care law. To those on the left who viewed him as an ideologue eager to pull the court to the right, the ruling will begin a re-examination of Chief Justice Roberts’s style and legacy, as it will for those on the right who considered the law unconstitutional and relied on him to make that point.

Graphic Siding With the Liberal Wing Since Chief Justice John G. Roberts Jr. joined the Supreme Court in 2005, there have been more than 100 5 to 4 decisions. In 30 of these decisions, the majority has included the court’s four most liberal members and another justice. Go to Feature »

Many scholars have said that Chief Justice Roberts is seeking 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 the chief justice, who was appointed to the court in 2005 by President George W. Bush, announce the upholding of the central legislative pillar of the Obama administration. He has said that he has wanted to restore the court’s reputation and reduce partisan language. But he was seen by many, at least on the left, as more devoted to conservative politics than to 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,” said Laurence H. Tribe, a liberal law professor from Harvard. 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 had charged. “I have some sense of gratification,” he said.

In the past, especially on campaign finance law but also on other socially sensitive issues like abortion and affirmative action, Chief Justice Roberts has not shied away from leading a conservative redraft of established law, causing some to accuse him of judicial activism.

But in this case, by referring to Congress’s power to impose a tax rather than a mandate, Chief Justice Roberts used the Obama administration’s backup argument about what makes the health care law constitutional.

If Mr. Bush ends up ruing the day he chose Mr. Roberts to lead the court, he would not be the first former president to do so. Mr. Bush’s father appointed Justice David H. Souter, who ended up as a mainstay of the court’s liberal wing. President Dwight D. Eisenhower was said to have called his appointments of Chief Justice Earl Warren and Justice William J. Brennan Jr. as two of his biggest mistakes.