Chief Justice John G. Roberts, seen Jan. 28, 2014 in Washington, cast the decisive vote in the 2012 Supreme Court decision that upheld the Affordable Care Act. (Larry Downing/Reuters)

Three years ago, a gruff-sounding Chief Justice John G. Roberts Jr. asked a question during the marathon hearings on the constitutionality of the Affordable Care Act about whether the mandate that individuals buy health insurance was really more like a tax on those who do not.

It got a little bit lost in the blizzard of words that accompanied the hearings, but turned out to be the foundation of Roberts’s opinion saving Obamacare.

Now, in the analysis of Wednesday’s King v. Burwell hearing, Roberts might be overlooked again.

That would be easy to do, because the chief justice was as taciturn at the oral arguments as he’s ever been since taking the center seat on the bench. He sat passively for most of the rapid-fire questioning and made only one inquiry of real substance, near the end of Solicitor General Donald B. Verrilli Jr.’s presentation.

“If you’re right — if you’re right about Chevron, that would indicate that a subsequent administration could change that interpretation?” Roberts asked.

The Affordable Care Act is facing another challenge at the Supreme Court in King v. Burwell, which deals with subsidies for health insurance. The case could cut out a major provision of Obamacare, causing the law to unravel. Here’s what you need to know about the case. (Julie Percha/The Washington Post)

Sounds like code. Could be very important. (Cornell law professor Michael Dorf calls the intense scrutiny of the oral argument transcripts “SCOTUS Kremlinology.”)

Roberts’s question was referring to “Chevron deference,” a doctrine mostly unknown beyond the halls of the Capitol and the corridors of the Supreme Court. It refers to a 1984 decision, Chevron U.S.A., Inc. v. Natural Resources Defense Council, Inc., and it is one of the most widely cited cases in law.

Boiled down, it says that when a law is ambiguous, judges should defer to the agency designated to implement it so long as the agency’s decision is reasonable.

It is, in fact, the basis for the lower court decision in King v. Burwell that the Supreme Court is considering.

The Internal Revenue Service decided that even though the Affordable Care Act provides tax credits for low- and middle-income Americans who buy insurance on exchanges “established by the state,” the law envisioned the same subsidies for those who buy on exchanges set up by federal authorities.

When a three-judge panel of the U.S. Court of Appeals for the 4th Circuit ruled in favor of the Obama administration, two of the judges said the law was contradictory.

“Simply put, the statute is ambiguous and subject to at least two different interpretations,” two of the judges wrote (the third said it was clear from reading the entire law that the subsidies were warranted).

They added: “It is thus entirely sensible that the IRS would enact the regulations it did, making Chevron deference appropriate.”

So one way to look at Roberts’s question is this: If the Obama IRS giveth, can the next president’s IRS taketh away?

The appeal of such a solution is easy to see from Roberts’s viewpoint, because it would push the decision-making back to the political branches.

Regardless of ideology, no justice is probably keen on taking away subsidies from an estimated 7.5 million people in 34 states without state exchanges. A decision based on Chevron deference could say to Congress: Fix the law to make it unambiguous. It says to the executive branch: Implementation of the law is up to you.

Dust off one of Roberts’s lines from his 2012 Obamacare decision: “It is not our job to protect the people from the consequences of their political choices.”

Not surprisingly, Verrilli pushed back — gently — on Roberts. He wants the court to find that, read in context, the law is not ambiguous.

“I think a subsequent administration would need a very strong case” to show that ending the subsidies “was a reasonable judgment in view of the disruptive consequences,” he said.

But as a practical matter, it is more important for the administration to win than to win in its preferred manner.

Besides, a new Democratic president would not change the decision. And a new Republican president’s decision to end the subsidies would be harder in 2017, when presumably even more Americans would rely on them.

The problem in Scotus Kremlinology is that there is no chance to review theories with the only people who can judge their plausibility. Perhaps Roberts was playing devil’s advocate, or perhaps his interest in Chevron means something else. There are plenty of folks who think the conservative justice will not save Obamacare twice.

Since Wednesday’s arguments, most of the tea-leaf reading has concerned Justice Anthony M. Kennedy. He was one of the four who wanted to find the entire act unconstitutional three years ago. Nonetheless, it was clear from the arguments his vote was in play now that the issue is how to interpret the law.

But as the last Obamacare decision showed, no one should ever forget about the chief justice, no matter how quiet he is.