Most Supreme Court watchers are fixated these days on Sebelius v. Hobby Lobby—the important challenge to the Affordable Care Act’s contraception mandate scheduled for argument Tuesday. And why wouldn’t they be? With its potent mix of religion, sex, Obamacare, and prayerful corporations, it’s the blockbuster case of the term. It is also a crucial test of Chief Justice John Roberts’s leadership on the Supreme Court.

Just two years ago, Roberts cast the deciding vote to largely uphold the Affordable Care Act. While the country remains divided over whether he acted like a traitor or a statesman, all would have to agree that, given the level of public scrutiny on the Court and the case’s overall importance (both substantively and to the President’s legacy), Roberts’s ACA vote was the defining moment of his tenure thus far. In a bold move, he broke ranks with his conservative colleagues, joined with the Court’s progressive wing, and preserved the President’s signature achievement. In Hobby Lobby, Roberts meets the ACA yet again, and the stakes for his reputation—and that of his Court—couldn’t be higher.

Chief Justice Roberts has often spoken about how important it is for the justices to maintain the legitimacy of the Court—by limiting divisive rulings, moving the law incrementally, and trying to stay above politics. For instance, in an interview with Jeffrey Rosen early in his tenure as chief justice, Roberts explained that the Court is “ripe for a ... refocus on functioning as an institution, because if it doesn’t it’s going to lose its credibility and legitimacy.” Expressing admiration for the great Chief Justice John Marshall, Roberts added that, even as a committed Federalist, Marshall preferred to move the law “in a way that ... wasn’t going to alienate people on the Court and turn the Court into another battleground.” While commentators certainly quibble over just how radical an effect John Roberts has had on the law—even Justice Antonin Scalia once attacked the chief justice’s approach in a pre-Citizens United campaign finance case as “faux judicial restraint”—there’s little question that Roberts himself prefers the image of the modest jurist to that of judge-as-hero (think Earl Warren) or judge-as-prophet (think Scalia).

He cultivated this image most dramatically in the first ACA case, joining with his progressive colleagues to uphold a Democratic president’s most important achievement—and in the middle of an election year, no less. Furthermore, just last term, the Roberts Court managed to reach an unlikely compromise in a blockbuster affirmative action case, and Roberts himself preserved the marriage-equality status quo in California with his majority opinion in Hollingsworth v. Perry. However, even in areas where Roberts has pushed the law dramatically to the right (like voting rights), he has tended to prefer a slower-moving, more incremental approach than his more radical colleagues, with seismic shifts (like Shelby County v. Holder) coming only after the political ground has already been prepared with previous, more modest decisions (like NAMUDNO v. Holder)—legal warning shots, if you will. He has also chipped away at progressive laws in a series of low-profile cases—for instance, those on the Court’s business docket. This strategy allows him to move the law to the right, while also preserving the institutional legitimacy of the Court.

Through this lens, Hobby Lobby presents a potential dilemma for the savvy Chief Justice. In the case, Hobby Lobby, a craft-store chain owned by Southern Baptists, is suing the government to seek religious exemption from the ACA's requirement that it offer insurance plans to employees that cover contraception at no extra cost. On the one hand, Roberts is confronting the ACA for the first time since the conservative firestorm over his decision largely upholding the Act. There’s little doubt that he’ll be tempted to throw conservatives a bone, siding with Hobby Lobby and against the ACA.