WASHINGTON — Chief Justice John Roberts has a favorite quotation from one of the giants who preceded him on the Supreme Court. Assessing the constitutionality of a law passed by Congress, Justice Oliver Wendell Holmes Jr. once wrote, “is the gravest and most delicate duty that this court is called on to perform.”

In finding a way to uphold President Barack Obama’s health care overhaul law on Thursday, Roberts performed the task with exquisite delicacy. That he did was a surprise from a judge whose rulings and background, including legal work in the administrations of President Ronald Reagan and the first President George Bush, suggested a conventionally conservative worldview.

To be sure, the chief justice considers himself the custodian of the Supreme Court’s prestige, authority and legitimacy, and he is often its voice in major cases. There was reason, then, to think he might have provided a sixth vote to uphold the law had Justice Anthony Kennedy joined the court’s four-member liberal wing. That would have allowed Roberts, the thinking went, to write a narrow, grudging majority opinion. But almost no one thought that he would provide the fifth vote, joining only the liberals, to uphold a Democratic president’s signal legislative achievement.

On the one hand, he said, the law’s requirement that most Americans obtain health insurance or pay a penalty could be justified under Congress’ power to levy taxes. The four liberals agreed, though they would have preferred to sustain the law as a regulation of commerce.

But the law could not be justified in that way, the chief justice went on, and here he was joined by the court’s four more conservative members.

Roberts suggested that even he did not find the tax argument especially plausible. But he quoted Holmes to explain why it was good enough. “As between two possible interpretations of a statute, by one of which it would be unconstitutional and by the other valid,” Holmes wrote, “our plain duty is to adopt that which will save the act.”

In the earlier cases, Roberts had drawn varying lessons from Holmes’ observation about the grave and delicate duty the Constitution imposes on the Supreme Court. Once, he said it counseled caution. Another time, he said it required action. On Thursday, he chose compromise, or perhaps statesmanship.

The last several chief justices each served for more than 15 years and participated in more than 1,000 decisions apiece. But just a handful of those rulings came to define their legacies. The court led by Chief Justice Earl Warren is remembered for its cases on desegregation and the rights of criminals. The one led by Chief Justice Warren Burger veered right in criminal cases but also identified a constitutional right to abortion. Under Chief Justice William Rehnquist, the Supreme Court delivered the 2000 election to George W. Bush.

The legacy of the Roberts court came into focus on Thursday, and it is one in which the chief justice serves as a sort of fulcrum and safety valve. He can pull the court back from bold action, as he did in 2009 when he persuaded seven of his colleagues to follow him in a novel interpretation of the Voting Rights Act rather than striking down its heart, as a majority of the justices had seemed inclined to do at the argument.

Or he could join it in bold action, as he did seven months later when he provided the fifth vote in Citizens United, which reversed precedents, struck down part of a major law and amped up the role of money in politics.

“There is a difference,” Roberts wrote in Citizens United, “between judicial restraint and judicial abdication.”

Thursday’s health care decision tacked back in the other direction, toward restraint.

That is not to say Roberts has ruled out aggressive action by the court, and he has said that he does not view striking down a federal law as of itself evidence of activism.

At Roberts’ confirmation hearings, Sen. Orrin Hatch, R-Utah, asked him whether “overturning a statute that we pass here in the national legislature is almost presumptively an example of judicial activism,” particularly “when saying that Congress has overstepped its bounds with respect to regulating interstate commerce.”

The nominee, then a federal appeals court judge, said no.

“If the court strikes down an act of Congress and it’s wrong, the court shouldn’t have done that,” Roberts said. “That’s not an act of judicial activism, it’s just being wrong.”

That statement foreshadowed a recent shift in conservative legal thinking about the role of the courts. It was not long ago that deferring to the elected branches was generally thought to be a positive virtue, an appealing sort of judicial modesty and restraint.

Lately, though, the columnist George Will has said the judicial branch “is dangerous to liberty when it is unreasonably restrained.” He criticized Mitt Romney, the presumptive Republican nominee, for his commitment to judicial restraint.

“One hopes Romney recognizes,” Will said, “that judicial deference to elected representatives can be dereliction of judicial duty.”

The health care decision provides a new data point. It will disappoint advocates of conservative and libertarian “judicial engagement,” and it will confound commentators who say there is nothing left to judicial restraint.

As Roberts explained at his confirmation hearings seven years ago, his approach to testing the constitutionality of federal laws involved significant deference to the elected branches.

“All judges are acutely aware of the fact that millions and millions of people have voted for you and not one has voted for any of us,” he told Hatch. “That means that you have the responsibility of representing the policy preferences of the people.”

He made the point more sharply on Thursday, in a part of his opinion in which he spoke only for himself.

“It is not our job,” he said, “to protect the people from the consequences of their political choices.”