WASHINGTON — The Affordable Care Act was back at the Supreme Court on Wednesday morning, facing a challenge that represents the biggest legal threat to the health care law since the justices three years ago upheld the requirement for individuals to have health insurance. The justices heard arguments Wednesday over the financial assistance available to help people purchase health insurance. Here are some key facts to get up to speed about the case, King v. Burwell.

1. The plaintiffs are four Virginians who argue that an IRS rule providing insurance subsidies in federal-run exchanges violates the text of the Affordable Care Act. The challenge, which is being funded by a libertarian think tank, argues the section of the ACA dealing with the subsidies only provides for aid in an exchange “established by the state” – and therefore, none of the nearly three dozen states with federal-run exchanges. The Obama administration and supporters say a full reading of the law makes clear that the subsidies can flow through federal-run exchanges, which they argue was the intent of Congress when they drafted the law.

2. Residents of 34 states that didn’t set up their own exchanges – including Maine – could lose their subsidies if the Supreme Court rules against the Obama administration. Residents of the remaining states running their own exchanges wouldn’t be affected.

3. About 8 million people would become uninsured if the Obama administration loses, according to Rand Corp. projections. Premiums would increase, on average, by 47 percent in the individual markets in those states, according to the analysis. Those projections assume that the subsidies are struck and neither Washington nor the states tried to rescue them.

4. There have been 55 friend-of-the-court briefs filed by outside groups in this case – 34 supporting the Obama administration and 21 supporting the challengers. Those on the government’s side include health insurers, hospitals, 22 states and senior Democrats who wrote the law. Those supporting the plaintiffs include seven states, 15 Republican lawmakers and a host of conservative and libertarian groups.

5. MIT economist Jonathan Gruber first earned notoriety because of this challenge. Opponents of the law last summer circulated a 2012 video of him saying – contrary to the administration’s argument – that states would deprive their residents of subsidies if they didn’t set up their own exchanges. Gruber, who had widely been described as an architect of the ACA, later said the comments were a mistake.

6. It’s unknown when the court will issue a decision, but the end of June seems to be a good bet based on ACA history. In 2012, the court issued a ruling upholding the individual mandate on the last day of that year’s term. In another health care case last year, the court ruled the government couldn’t require closely held businesses to provide contraceptive coverage to employees against their religious beliefs. This year, it’s possible that the court could issue landmark rulings on health care and gay marriage on the same day.

– The Washington Post