Pushing his colleagues to join an opinion or issue a unanimous opinion in particularly controversial cases may also fall within the chief justice's responsibility. Chief Justice Earl Warren famously campaigned among his brethren for months, mindful that less than a unanimous decision in Brown v. Board of Education would even further fracture the country and give Southern segregationists a toehold for their resistance to integration. The Little Rock Nine case, Cooper v. Aaron, was yet another instance in which it was important that all of the justices stand behind the chief's articulation of the court's power.

The importance of unanimous decisions in politically contentious cases has been recognized by the court under other chiefs since Warren. The Nixon-Watergate tapes case and even Clinton v. Jones were unanimous decisions of the court. If one reads the transcript of the oral argument in Clinton v. Jones and hears the deep skepticism expressed by both Justice Ginsburg and Justice Breyer about the prospect of allowing civil suits against a sitting president, it is apparent that something happened between the oral argument and the release of the decision to produce a unanimous decision allowing the Jones suit to go forward.

The nadir of Justice Rehnquist's tenure as chief was Bush v. Gore. Derision heaped on the court's decision because the case installed the Republican candidate as president. In addition, because of the embarrassingly thin reasoning of the court's opinion, the ridicule paled in comparison to the deep suspicion engendered by the fact that the 5-4 decision split along party lines. The sense of betrayal that accompanied that 5-4 decision -- from both liberals and conservatives alike -- should have served as a cautionary tale to Roberts.

Instead, he has been singularly unsuccessful in exercising the kind of leadership that presses the justices on the court to reach decisions reached by a larger consensus than that of the entrenched conservative majority. Thus, it came as no surprise two weeks ago, when it was revealed that the Supreme Court's popularity rating had dropped to an all-time low of 44 percent.

That is, until this week's health care decision. Justice Roberts' decision to break with his conservative compatriots to uphold the Affordable Care Act, given his track record as chief, is a monumental step forward in leadership on the court. The public's sense that the court is politically captured and rigidly ideological has been reinforced by decisions in which the four conservative justices all ruled together or in which the four centrist justices rule together in politically controversial cases. Every time a justice from either the conservative or centrist camp departs from their perceived ideological home to rule in a way that is surprising and unpredictable, it bolsters the legitimacy of the court in the eyes of the public. That's why the inclusion of justices from across the ideological spectrum in support of the majority decision in the Arizona immigration case (pdf) -- despite some of the troubling aspects of the court's decision -- also gave the court an institutional legitimacy boost last week.

By all means we should give credit where credit is due. Without question, President Obama should be pleased and even grateful that Chief Justice Roberts saved the health care law. But we should also remember that in so doing, the chief justice of the United States saved both the legitimacy of the court as well as his own.

Sherrilyn A. Ifill is a professor of law at the University of Maryland School of Law and a civil rights lawyer.

Correction: A previous version of this story stated that the Supreme Court's popularity rating was 28 percent instead of 44 percent. We regret the error. We have also clarified that justices from across an ideological spectrum (not only Justice Sotomayor), who supported the majority decision in the Arizona immigration case, gave the court a legitimacy boost last week.

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