Richard Wolf

USA TODAY

WASHINGTON — The most serious challenge to President Obama's health care law since it survived the Supreme Court by a single vote in 2012 isn't a balky website, public opinion or the Republican takeover of Congress. It's the Supreme Court — again.

In a case likely to be heard in March and decided in June, the justices will dissect the meaning of four words on page 95 of the 906-page Patient Protection and Affordable Care Act — four words that could render health insurance premiums unaffordable for millions of Americans.

Here's a look at the issues in King v. Burwell:

Question: Why the fuss over four words?

Answer: The law states that tax credits will be available through so-called exchanges, or online marketplaces, "established by the State." When it was being crafted, it was assumed that all 50 states would create their own exchanges. After it passed in March 2010, it became clear that many states would rely on the federal government to operate them, as the law allows.

In 2012, the Internal Revenue Service made the subsidies available in all states. The law's challengers claim they cannot be offered in exchanges operated by the federal government. Thirty-six states fit into that category. Without subsidies, insurance costs would skyrocket.

Q: What's more important — the plain meaning of those words, or the context?

A: That depends on whom you ask. The challengers say the plain meaning is clear: Subsidies can be offered only in state exchanges. If that's what Congress wrote, they say, the Supreme Court cannot change it, and the IRS cannot extend tax credits to federal exchange customers.

The administration and other proponents say the words must be read in the context of a law clearly intended to make health insurance more widely available and affordable. Even the suspect words appear in a subtitle of the law that reads, "Affordable Coverage Choices for All Americans."

Q: Do the challengers have other arguments?

A: To back up their claim that Congress meant what it said — that it wasn't simply a drafting error — the law's opponents say lawmakers purposely made tax credits available only in state-run exchanges as an incentive for governors and legislatures to create their own exchanges. Otherwise, the argument goes, their residents would get cheated out of a major benefit.

Q: Do proponents have other arguments?

A: They emphasize how statutes customarily are interpreted by courts. When a law's text would lead to an absurd result, they say, it requires reinterpretation. When it's ambiguous, they say, courts should defer to administrative agencies' interpretations — a canon known as "Chevron deference" after a 1984 case involving the energy corporation.

Q: What has happened in the courts until now?

A: The Supreme Court upheld Obamacare in 2012 by a 5-4 vote in an opinion written by Chief Justice John Roberts. Last year, it exempted closely held companies with religious objections from a follow-up regulation that insurance plans must include full coverage for contraceptives.

The argument over subsidies in federal exchanges is the focus of four separate cases. In King, the U.S. Court of Appeals for the 4th Circuit in Richmond unanimously upheld the law. A panel of the D.C. Circuit appeals court ruled 2-1 for the challengers, but that opinion was vacated by the full court pending further review, which is on hold. A district court in Oklahoma ruled in favor of the challengers. A fourth case is pending in Indiana.

Q:Why is the Supreme Court jumping in now?

A: Technically, there is no split among appellate courts, which usually prompts the high court's intervention. The court occasionally takes on cases of major national importance before such a split develops, although last month, it refused to do that in the case of same-sex marriage.

One reason probably has to do with the time clock. In their brief, the challengers argue that the longer the law is in effect, the greater the risk for people who receive subsidies that could be "clawed back."

Another reason could be purely ideological. Proponents contend that the four conservative justices who opposed the law from the start probably were the ones voting to hear the case.

Q: What happens if the challengers win?

A: A chain reaction, beginning with the impact on premiums. More than 7 million people could lose subsidies in 2016, according to the non-partisan Urban Institute. On average, the subsidies pay more than 75% of premium costs, or about $4,700 per year.

Without subsidies, more than 8 million people would be exempt from the requirement that individuals purchase insurance, because it no longer would be affordable. Hundreds of thousands of employers would be exempt from penalties for not covering employees, because their workers would not be subsidized in states with federally operated exchanges.

Q: Would that kill the law?

A: Not necessarily. First of all, individuals in 14 states that operate their own exchanges would still be eligible for tax credits. Additional states, such as those with Democratic governors or with Republican governors who have accepted increased Medicaid funding — another of the law's carrots — might set up exchanges.

Other sections of Obamacare would remain intact, at least initially. Those include a guarantee that insurance companies cover everyone, the Medicaid expansion, Medicare cost containment measures and allowing young people under 26 to remain on their parents' policies.

Q: What's likely to happen?

A: The law probably has at least four votes on the court — those of Justices Ruth Bader Ginsburg, Stephen Breyer, Sonia Sotomayor and Elena Kagan. Whether the four justices who dissented in 2012 — Antonin Scalia, Anthony Kennedy, Clarence Thomas and Samuel Alito — buy the challengers' plain-text argument is unclear. But it takes four votes in private conference to hear the case, and they are the most likely antagonists.

That could leave Roberts where he found himself two years ago — on the spot. Because the court historically is known by its chief justice, he knows the "Roberts Court" — entering its 10th year — will be remembered at least in part for its battles with Obama over his most cherished domestic policy achievement.