I've posted previously about the case (here). Briefly, the IRS treats federal exchanges and state exchanges identically for purposes of ACA subsidies. The challengers argue that this treatment misinterprets the phrase "an Exchange established by the State" in the Affordable Care Act. If the Court sides with the challengers, the bill's supporters believe that the consequences would be devastating.
The Supreme Court posted the oral-argument transcript earlier today. Other sites have done an expert job of covering the argument, including Dahlia Lithwick for Slate, Adam Liptak for The New York Times, Lyle Denniston and Amy Howe for SCOTUSBlog, and Jonathan H. Adler for The Volokh Conspiracy. So I'll keep my comments brief.
Reading the tea leaves
The consensus seems to be that Justices Ginsburg, Breyer, Kagan, Sotomayor are firmly in favor of the government, while Justices Scalia, Thomas, and Alito are with the challengers. Chief Justice Roberts (who was the "swing" vote in the previous challenge to the Affordable Care Act) asked almost no questions.
Justice Kennedy made an important observation about federalism that leads some commentators to think he's not a clear vote for the challengers (although others disagree).
Transcript, page 16
Justice Kagan asked the challengers a question that involved three hypothetical law clerks - Amanda, Elizabeth, and Will.
Transcript, page 9
But Justice Kagan actually has four law clerks, and she gave the names of three of them in her hypothetical. Odd that the fourth clerk didn't make a cameo - perhaps he came up with the question in the first place.
When the government argued that a pro-challenger decision would have dire results, Justice Scalia was skeptical. One might reasonably think that the statute Congress actually passed is ambiguous. Congress could fix the problem itself. But Solicitor General Donald Verrilli (arguing for the government) went realpolitik, adding some well-received levity.
Transcript, page 54
This is one of the most important cases the Supreme Court took for this term - I wouldn't expect an opinion to get issued until late June or early July.