WASHINGTON (AP) — The U.S. Supreme Court appeared sharply divided over President Barack Obama's health care law Wednesday, aggressively questioning both sides in a case that threatens insurance coverage for millions of people.

The questions came during oral arguments in the latest challenge to Obama's attempt to bring universal health care to the United States, a central part of his legacy. Republicans claim the law infringes on individual liberties by requiring almost everyone to have insurance and results in Americans paying more and getting shoddy care.

Chief Justice John Roberts said almost nothing in nearly 90 minutes of back-and-forth, and Justice Anthony Kennedy's questions did not suggest how he will come out.

Of the nine justices, Roberts and Kennedy are seen as particularly pivotal in this case. Roberts, a conservative, was the decisive vote to uphold the law in 2012.

Otherwise, the same liberal-conservative divide that characterized the earlier case was evident Wednesday.

The justices are being asked to determine whether the Affordable Care Act makes people in all 50 states eligible for federal tax subsidies that make the cost of health insurance premiums affordable.

The challengers argue that the only people eligible for the subsidies are those who live in states that created their own health insurances marketplaces — exchanges intended to allow people to find affordable coverage if they don't get it through their jobs or government programs for the poor, elderly and veterans.

If the Supreme Court agrees with the plaintiffs, it would have dramatic consequences because roughly three dozen states, many run by Republican governors, opted against creating their own marketplace. Instead, residents of these states rely on a federally run exchange. Independent studies estimate that 8 million people could lose insurance coverage if the lawsuit prevails.

"This case is no less important to the future of the Affordable Care Act" than the court's decision in 2012, said Erwin Chemerinsky, dean of the University of California at Irvine law school.

While Democrats dread losing the case, there's a risk for Republicans too: millions of voters in Republican-governed states would lose their subsidies. Many of those states have Republican senators up for re-election in 2016.

Activists on both sides were in place outside the marble courthouse by 5:30 a.m. Wednesday. Supporters of the law outnumbered opponents. Some held placards showing how many people in each state would lose insurance if the court rules that the law does not allow subsidies everywhere.

The case focuses on four words — "established by the state" — in a law that runs more than 900 pages. The challengers say those words are clear and conclusive evidence that Congress wanted to limit subsidies only to people in states that set up their own exchanges. Those words cannot refer to exchanges established by the federal government, the challengers argue.

The Obama administration, congressional Democrats and 22 states argue that it's absurd to suggest Congress would have constructed the law the way that undermines its own program. They say the full law makes clear there is no such distinction between federal and state exchanges. But with Republicans controlling Congress, Democrats have little hope of tweaking the law's language.

The idea behind the law's structure was to decrease the number of uninsured. The law prevents insurers from denying coverage because of "pre-existing" health conditions. It requires almost everyone to be insured, so that healthy people can offset the costs of the sick, and provides financial help to consumers who otherwise would spend too much of their paycheck on their premiums.

Both sides in the case argue that the law unambiguously supports only its position. So far, lower courts have been split on the issue. A Supreme Court ruling is expected by late June.

Three years ago, opponents of the Affordable Care Act failed to kill the law in an epic, election-year Supreme Court case. Roberts joined with the court's liberal justices and provided the crucial vote to uphold the law in the midst of Obama's re-election campaign.

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Associated Press writer Mark Sherman contributed to this report.