This article is from the archive of our partner National Journal

Obamacare's future rests with one of the Supreme Court's conservative justices. But no one is quite sure which one.

The health care law's insurance subsidies—and, by extension, its fundamental structure—can't survive unless one Republican-appointed justice votes to preserve them. But lawyers on both sides of the case are divided over who is more likely to cast that vote: Chief Justice John Roberts, or Justice Anthony Kennedy.

Kennedy, of course, has the reputation as the Court's traditional swing vote. Yet it was Roberts who cast the deciding vote to uphold Obamacare in 2012. With oral arguments less than a month away, the law's challengers aren't sure whom they ought to be more worried about this time—and its supporters aren't sure whom they are most likely to win over.

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Making things more difficult: In King v. Burwell, the case challenging Obamacare's subsidies in most of the country, the arguments most likely to appeal to Roberts and the arguments most likely to appeal to Kennedy aren't necessarily the same.

"If you were thinking about this from the lawyers' perspective, it would be a mistake to take either the chief or Justice Kennedy for granted," said Kevin Walsh, a law professor at Richmond University and a former clerk for Justice Antonin Scalia.

When the case first reached the high court, the popular assumption among legal experts was that it would fall along the same lines as the 2012 case over the individual mandate: four locked-in votes on each side, with Roberts caught in the middle.

The broad strokes of that assumption are easy enough to understand: The chief justice's 2012 decision was, in no small part, a reflection of his concern for the Supreme Court as an institution. A ruling against the administration in King would have many of the same political and practical effects that Roberts was trying to avoid back then: It would wreak havoc on real-world insurance markets, as well as the signature domestic achievement of a sitting president.

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Furthermore, unlike 2012, millions of people now have insurance through the law's exchanges and are receiving the subsidies at issue in King. Roberts already saved Obamacare once—why would he cripple it now, after it has started to work?

But the King case is much different from the challenge to the individual mandate, in ways that could make it easier for Roberts to side with the challengers.

Most important, it's not a constitutional question. It's about interpreting the text of the statute—an area of law that increasingly belongs to conservatives, including Roberts.

"Where Roberts tends to trim his sails is where a ruling would really tie Congress's hands," said Jonathan Adler, a law professor at Case Western Reserve University who has been instrumental in the case against the subsidies. "Roberts shies away from broad constitutional questions because he doesn't want to take power away from Congress. He would much rather say, 'Congress, you screwed up; try again,' than, 'Congress, you can't do that at all.' "

That's precisely the dynamic that has many liberals worried about King. They're afraid the justices will rule, essentially, that there's a glitch in the Affordable Care Act, but that it's not their job to fix it. Such a ruling would be extremely damaging to Obamacare, but might seem entirely consistent with Roberts's version of judicial minimalism.

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"It would be a mistake to simply assume the justices will vote the same way they did on [the individual mandate]," Walsh said.

And that means Kennedy could be in play this time.

Kennedy didn't get much attention at the outset of King v. Burwell, mainly because he was seen as irretrievably hostile to Obamacare. The dissent he joined in 2012 was blistering, arguing that the entire law—not just the individual mandate—should have fallen. And Kennedy reportedly resisted aggressive lobbying by Roberts to change his vote.

But the same factors that could change Roberts's analysis could also change Kennedy's.

"I think Kennedy is the more likely swing vote," Adler said.

Statutory interpretation and textualism are important to conservatives like Roberts and Scalia, but that's never been Kennedy's wheelhouse. He's much more likely to make his mark on constitutional questions—and to consider broader issues, like the overarching purpose of a law, in statutory interpretation cases. He might be open to the Justice Department's argument that Congress intended for subsidies to be available in both state-run and federally run exchanges, irrespective of that "established by the State" language.

"His approach to text is maybe not as consistent as some of the other justices," Adler said.

The balance of state and federal power is also a pet issue of Kennedy's, and some of Obamacare's allies have played up a federalism argument in their briefs to the Court. A group of state governments, in an amicus brief in King, argued that a ruling invalidating the subsidies would upend the deal states thought they were making.

States always assumed that their residents would get subsidies no matter which level of government set up their exchanges, the states argue—some of them said so explicitly as they laid out their decision not to proceed on their own. If Congress meant to deny subsidies to residents of those states, it should have said so more clearly, they argue.

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"Congress did not give States clear notice that their citizens would be punished and their insurance markets ruined if the State chose [a federal exchange]," the states wrote.

Even lawyers on the other side of the case say that argument could work—maybe with Roberts, and more probably with Kennedy.

"My view on those is that they're serious arguments. They're intended to appeal to the chief and/or Justice Kennedy, I don't find them convincing under current precedent. "¦ [But] I think there are some justices on the Court who are open to moving the law in that direction," Adler said.

Not everyone is so sure Kennedy is up for grabs, though.

Some liberal advocates worry privately that he's simply too predisposed against Obamacare. And, while the Justice Department's brief—the one that counts—includes those federalism arguments, its primary emphasis is on the text and purpose of the statute, arguing that, read as a whole, the law clearly treats state and federal exchanges the same.

Harvard professor Lawrence Tribe, who recently published a book on the Roberts Court, believes Roberts might be open to that interpretation.

"A lot of conservatives say, 'The text is clear, it's about an 'exchange established by the state.' Roberts is not going to find it that simple. The text is at least ambiguous. It uses terms like 'such exchange' when referring interchangeably to an exchange that a state sets up itself and to the federal exchange a state might choose to treat as its own," Tribe told The Washington Post.

Roberts—not Kennedy—is probably the key to King, Tribe said in the same interview.

"I think it's very likely he will be. But it's possible that if he votes to uphold the administration's position, Justice Anthony Kennedy could conceivably join him," Tribe said. "But I think if he does not vote to uphold the administration's position, then it's almost inconceivable that it would be upheld."

This article is from the archive of our partner National Journal.

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