The court’s job in the health care case, he wrote on Thursday, was to let the political process work and ensure that Congress’s intentions were honored. “In a democracy, the power to make the law rests with those chosen by the people,” he wrote.

The court’s job in the marriage case was the same, he said on Friday. “It can be tempting for judges to confuse our own preferences with the requirements of the law,” he wrote. “The majority today neglects that restrained conception of the judicial role. It seizes for itself a question the Constitution leaves to the people, at a time when the people are engaged in a vibrant debate on that question.”

The two cases illuminate the puzzle that is Chief Justice Roberts. When President George W. Bush nominated him in 2005 to replace Justice Sandra Day O’Connor, his work as a lawyer in Republican administrations and his brief record as an appeals court judge gave every indication that his would be a reliably conservative vote.

But then Chief Justice William H. Rehnquist died, and President Bush nominated Judge Roberts to take over leadership of the court. Taking the center seat at the Supreme Court may have moved him toward the ideological center, too, if only a little.

The chief justice has, after all, institutional responsibilities along with jurisprudential ones. He is the custodian of the Supreme Court’s prestige, authority and legitimacy, and he is often its voice in major cases.