A group of pro-Obamacare states argued in a brief to the Supreme Court that, when they made their decisions about setting up a state-run exchange, no one told them their decision could deny subsidies to their residents and throw their insurance markets into chaos. And that would have been pretty useful information.
The argument is aimed at Roberts, who has said previously that states deserve "clear warning" of the strings Congress attaches to their decisions; and to Kennedy, who's seen as more likely than his fellow Republican appointees to base his decision on factors other than the text. Jonathan Adler, one of the conservative legal scholars who helped make this challenge happen, has said he takes the federalism argument seriously.
Is the law ambiguous?
Both sides argue that the law is clearly on their side. But what if it's not?
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There's a way for the challengers to win if the justices decide the law is unclear, but the government has the better odds. In many cases, when statutes are unclear, the courts defer to the interpretation of the agencies implementing them. That principle, known as Chevron deference, has been around for a long time, and it was the basis on which a lower court upheld Obamacare's subsidies nationwide.
To have their strongest day, the challengers in King would need to persuade the court that the law clearly and unambiguously limits subsidies only to certain states. If the court seems to think the statute is unclear, it'll be a question of how it settles that ambiguity: through Chevron deference; or by saying, in effect, the text says what it says, even if it wasn't meant to.
What about the practical implications?
As a policy matter, the subsidies are part of a "three-legged stool" designed to make the Affordable Care Act work. Wipe out one leg of that stool, and the whole system could begin to tip over. Similarly, Obamacare is real now: About 11 million people have coverage through the law's exchanges, and the majority of them would lose that coverage if the court sides with King.
None of that matters, legally. But in a case that comes down to parsing one small phrase versus another, legal experts say, there's plenty of room for other, non-legal considerations.
They could cut either way: Some liberals worry that Kennedy has simply made up his mind about Obamacare and won't be especially open to arguments he might otherwise find persuasive. On the other hand, since Kennedy often is willing to think more about the real-world effects of his rulings, some analysts think the prospect of actually taking away people's health care—a reality that didn't exist yet in 2012—will push him away from a narrow reading of the text.
Does standing matter?
Standing—the legal right to bring a lawsuit—is another issue most likely to matter as a sort of subtext, but legal experts following the case are eager to see whether it comes up at all during oral arguments.