(John Jay/For The Washington Post)

Robert Barnes is The Washington Post’s Supreme Court correspondent.

The first time the Affordable Care Act came before the Supreme Court, its constitutional foundation under attack, John G. Roberts Jr. was its unlikely savior. In a spectacular display of spot-welding, the chief justice joined fellow conservatives on some points and brought liberals on board for others. Roberts was the only member of the court to endorse the entire jerry-rigged thing, and even he made sure to distance himself from the substance of the law. (“It is,” he wrote, “not our job to protect the people from the consequences of their political choices.”) Still, his efforts rescued President Obama’s signature achievement on grounds that many had dismissed as an afterthought.

As long as Justice Anthony M. Kennedy is on the court, he will most often be the decider when the justices split along their familiar ideological fault lines. But, slowly and quietly, Roberts is the one trying to build its legacy. He sees it as somehow exempt from the partisan fugue that long ago enveloped Washington. Justice Stephen G. Breyer has worried that the public might see him and his colleagues as “nine junior-varsity politicians”; public approval of the Supreme Court is falling. But while all of the justices bristle at the notion of a political court, the eponymous head of the Roberts court has the most to lose. After all, its decisions cannot be respected if the court is not respected. “It is a very serious threat to the independence and integrity of the courts to politicize them,” Roberts said at his 2005 confirmation hearings.

Roberts, 60, jokes about the “odd historical quirk” that gives the chief justice only one vote. But he has learned to use the tools that come with the job: He shapes the discussion at conference; he writes the court’s opinion, or assigns it strategically, when he is in the majority; he’s happy to settle for nonthreatening, incremental changes that may bloom later into something more. And last term, what Roberts has described as the chief justice’s “particular obligation to try to achieve consensus” paid off. The share of unanimous decisions soared to 66 percent, a level not seen since the 1940s. The share of 5-to-4 decisions, high during Roberts’s tenure compared with those of other chief justices, fell to 14 percent, the lowest since he joined the court.

And then here comes Obamacare II. In King v. Burwell , to be argued Wednesday, plaintiffs say the text of the law must be interpreted in a way that would neuter it, canceling health insurance subsidies for about 7.5 million Americans in at least 34 states. Can Roberts’s portrayal of the Supreme Court as above politics survive another round with the most partisan issue of the decade?

Roberts, of course, has not ceased to be a conservative. Before Obamacare — or since — it’s hard to think of a case in which he has not voted the way conservative activists had hoped when they recommended him to President George W. Bush. The Roberts court has been described as the most pro-business in history. Its liberals complain that consumers are on a losing streak and that the court has imposed new roadblocks for those trying to prove discrimination.

And when the issue is important to the chief justice, or when there seems to be no chance for compromise, he has been decisive. In a suite of cases, for instance, the court has systematically dismantled campaign finance restrictions, calling them hostile to free speech rights — Citizens United v. Federal Election Commission being the most famous of several 5-to-4 votes. Roberts wrote the majority opinion in the most recent, which featured the Republican National Committee as a plaintiff.

He has also forcefully opposed the government’s use of racial classifications, and he wrote to strike down a key provision of the Voting Rights Act. Roberts, unlike Kennedy or even Justice Antonin Scalia, had never been the lone conservative to join the court’s liberals to make a majority in a major case — until the Affordable Care Act.

Yet chief justices tend not to see the court as a vehicle for advancing their unadulterated ideas. A transformation came over Roberts’s predecessor and mentor, William H. Rehnquist, after becoming chief. As an associate justice, Rehnquist wrote so many solo dissents that his clerks awarded him a Lone Ranger doll. But like chief justices before him, in his new role he felt a responsibility to guard the court’s precedents and image. The most famous example is his metamorphosis on the Miranda rule, requiring police officers to read suspects their rights. For years, Rehnquist had denounced the rule as constitutionally unsound. But after his ascent he voted to uphold it, saying the warnings “have become part of our national culture.”

Roberts came to the court as chief, so there is no similar evolution to judge. But those who know him say his vision of the responsibilities of the chief is paramount. “Associate Justice Roberts would be much closer to Scalia than Chief Justice Roberts is in terms of their approach to cases,” says one lawyer, who spoke on the condition of anonymity because he argues Supreme Court cases. Anticipating this approach, Roberts pledged himself to the principle of stare decisis at his confirmation hearings and preached a gospel of judicial modesty, saying he came with “no agenda.”

While he can’t change Washington’s partisan warfare, Roberts does what he can to avoid becoming a weapon in it, those who know him say. Unlike his colleagues, he does not give interviews. He avoids partisan gatherings such as the Federalist Society’s annual gala (where Scalia and Justices Clarence Thomas and Samuel A. Alito Jr. take turns providing the after-dinner remarks) or the American Constitution Society (where Justices Ruth Bader Ginsburg and Sonia Sotomayor have been honored). Even the bipartisan Gridiron Dinner, where the nation’s political elite gather for gentle roasts, may have proved too much for Roberts: There’s always a seat at the head table for the chief justice of the United States, but this one hasn’t attended since 2009.

His protection of the court’s image extends to his colleagues. In the first Affordable Care Act case, conservative activists and pols demanded that Justice Elena Kagan recuse herself because of her work as Obama’s solicitor general, while liberal groups called for Thomas to step aside because his wife is a prominent conservative activist. Roberts, without specifically mentioning either, tried to shut down those complaints in his annual “state of the judiciary” report that year. “I have complete confidence in the capability of my colleagues to determine when recusal is warranted,” he wrote. “They are jurists of exceptional integrity and experience whose character and fitness have been examined through a rigorous appointment and confirmation process.”

But the consensus compulsion is clearest when the court issues narrow opinions. With Obamacare, for instance, Roberts bobbed and weaved, agreeing with conservatives that Congress had exceeded its power to regulate interstate commerce but also siding with liberals to save the individual mandate by calling it a tax. In the last term, narrow rulings made it possible for the court to unite, at least on the bottom line, in striking down abortion-clinic buffer zones in Massachusetts and rejecting efforts to restrict class-action suits alleging security fraud.

His efforts are not universally revered. “John Roberts, Thy Name Is Traitor,” one conservative news site blared after the Obamacare decision. Glenn Beck offered T-shirts with Roberts’s smiling face and the word “COWARD” underneath. John Yoo, a former Justice Department official in the George W. Bush administration, wrote that this coming week’s challenge gives Roberts a chance to “atone.” Scalia, a frequent Roberts ally, once mocked Roberts’s “faux judicial modesty.” And in last term’s case on the buffer zones, he dismissed the chief’s majority opinion as “Something for Everyone.”

Roberts insists that the nation’s poisonous political climate, rather than anything the justices have done, is to blame for damage to the court’s reputation. But he lives in a universe with a gridlocked Congress, a combative president who calls out the court at his State of the Union address and colleagues identified as often by party affiliation as by their judicial philosophies. “Today’s partisan split, while unprecedented, is likely enduring,” says a study of the court by professors Neal Devins of William & Mary and Lawrence Baum of Ohio State. “A five-member Democratic Court will reach sets of decisions that are quite different from those of a five-member Republican Court.”

Perhaps then political tensions are simply inescapable. But paradoxically, the court is hard to characterize precisely because of its close balance. The Roberts court has found rights for Guantanamo detainees; ruled that the Second Amendment secures an individual right to keep guns; handed gay rights activists some of their most important victories; drastically cut back on the ability of legislatures to restrict campaign contributions; and narrowed the options for punishing juvenile defendants. Alternating liberal and conservative victories on this term’s two most important cases — gay marriage and health care — offer similar chances for “balance” when the justices complete their work this June.

The court’s conflicting messages usually depend on Kennedy siding either with the conservative justices or the liberal ones. But about two-thirds of the time, the Reagan appointee finds his natural home on the right. And while such snapshots can be misleading, in the last term Roberts agreed with Kennedy more than any other justice did. Kennedy was on the winning side in more opinions than any of his colleagues; Roberts was right behind.

This hardly means that either side sees the court as a neutral arbiter. Most liberal analysts praised Roberts’s first Obamacare decision, but some forecast that his reasoning would be used to secure conservative victories in the future. Others speculated that it gave Roberts “cover” to move on to issues about which he felt more strongly. They felt vindicated a year later when he sided with his conservative colleagues and wrote the 5-to-4 opinion striking down a crucial component of the Voting Rights Act.

Roberts appears to know this. At an appearance at the University of Nebraska law school last fall, he talked about how gridlock between Obama and congressional Republicans means collateral damage for the nine justices, too. “I don’t want it to spill over and affect us,” he said. “We are not Democrats and Republicans in how we go about it, and in nine years I’ve never seen any political issue like that arise between us.” He worried that the near-party-line confirmation of “somebody as eminently qualified as our newest member,” Kagan, could suggest that people perceive the court, too, as a political body.

It is a perception that, despite Roberts’s zealous labors, the court could soon reinforce, at least on the left. “A 5-to-4 decision invalidating the premium tax credits” in King v. Burwell “would seriously call into question the legitimacy of the court,” Tim Jost, a Washington and Lee University law professor and health-care expert, said in a video distributed by the liberal Center for American Progress. “I think it’s going to be [decided] pretty transparently for political reasons.”

Read more from Outlook:

Five myths about King v. Burwell

“Jihadi John,” a graduate of my radical university

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