WASHINGTON — The Supreme Court is hearing arguments in a major test of President Barack Obama’s health overhaul that threatens insurance coverage for millions of people.

The justices are meeting Wednesday to try to determine whether the law makes people in all 50 states eligible for federal tax subsidies to cut the cost of insurance premiums. Or does it limit tax credits only to people who live in states that created their own health insurance marketplaces?

A ruling that limits where subsidies are available would have dramatic consequences because roughly three dozen states opted against their own marketplace, or exchange, and instead rely on the U.S. Health and Human Services Department’s healthcare.gov. Independent studies estimate that 8 million people could lose insurance coverage.

Opponents of the Affordable Care Act failed to kill the law in an epic, election-year Supreme Court case in 2012. Chief Justice John 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.

The new case, part of a long-running political and legal fight to get rid of the law also known as Obamacare, 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 those consumers who get their insurance through a marketplace, or exchange, that was established by the state.