Three years after the Supreme Court first heard a challenge to the Affordable Care Act, the justices today hear a new lawsuit challenging financial assistance provided through the law. (Photo by Alex Wong/Getty Images)

Obamacare is back at the Supreme Court this 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 will hear arguments today 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.

1The plaintiffs are four Virginians who argue 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.

2Residents of 34 states that didn't set up their own exchanges could lose their subsidies if the Supreme Court rules against the Obama administration. Residents of the remaining states running their own exchanges, including Maryland and the District of Columbia, wouldn't be affected. Here are the states with federal-run exchanges:

3An estimated 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.

4There 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.

5MIT 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.

MIT professor Jonathan Gruber, a high-profile consultant on the ACA, has apologized for previous controversial comments. (AP Photo/Molly Riley)

6It'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 last day of that year's term. In another Obamacare 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 Obamacare and gay marriage on the same day.

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)