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The Affordable Care Act is once again before the Supreme Court.

On March 4, the justices will hear oral arguments in , a case challenging the validity of tax subsidies helping millions of Americans buy health insurance if they don’t get it through an employer or the government. If the court rules against the Obama administration, those subsidies could be cut off for everyone in the three dozen states using , the federal exchange website. A decision is expected by the end of June.

Here are five things you should know about the case and its potential consequences:

1: This case does NOT challenge the constitutionality of the health law.

The Supreme Court has already found the Affordable Care Act is constitutional. That was settled in 2012’s .

At issue in this case is a stipulating that subsidies are available to those who sign up for coverage “through an exchange established by the state.” In issuing in 2012, however, the IRS said that subsidies would also be available to those enrolling through the federal health insurance exchange. The agency noted Congress had never discussed limiting the subsidies to state-run exchanges and that making subsidies available to all “is consistent with the language, purpose and structure” of the law as a whole.

Last summer, the U.S. Court of Appeals for the Fourth Circuit in Richmond ruled that the of the law. While the three-judge panel agreed that the language in the law is “ambiguous,” they relied on so-called “a legal principle that takes its name from a 1984 Supreme Court ruling that held that courts must defer to a federal agency’s interpretation as long as that interpretation is not unreasonable.

Those challenging the law, however, insist that Congress intended to limit the subsidies to state exchanges. “As an inducement to state officials, the Act authorizes tax credits and subsidies for certain households that purchase health insurance through an Exchange, but restricts those entitlements to Exchanges created by states,” wrote Michael Cannon and Jonathan Adler, two of the fiercest critics of the IRS interpretation, in an in the Health Matrix: Journal of Law-Medicine.

In any case, a ruling in favor of the challengers would affect only the subsidies available in the states using the federal exchange. Those in the 13 states operating their own exchanges would be unaffected. The rest of the health law, including its expansion of Medicaid and requirements for coverage of those with pre-existing conditions, would remain in effect.

2: If the court rules against the Obama administration, millions of people could be forced to give up their insurance.

A study by the found that if subsidies in the federal health exchange are disallowed, 9.3 million people could lose $28.8 billion of federal help paying for their insurance in just the first year. Since many of those people would not be able to afford insurance without government help, the number of uninsured could rise by 8.2 million people.

A from the Urban Institute looked at those in danger of losing their coverage and found that most are low and moderate-income white, working adults who live in the South.

3: A ruling against the Obama administration could have other effects, too.

Experts say disallowing the subsidies in the federal exchange states could destabilize the entire individual insurance market, not just the exchanges in those states. Anticipating that only those most likely to need medical services will hold onto their plans, insurers would likely increase premiums for everyone in the state who buys their own insurance, no matter where they buy it from.

“If subsidies [in the federal exchange] are eliminated, premiums would increase by about 47 percent,” said Christine Eibner of the RAND Corporation, who co-authored a projecting a 70 percent drop in enrollment.

Eliminating tax subsidies for individuals would also impact the law’s requirement that . That’s because the penalty for not providing coverage only kicks in if a worker goes to the state health exchange and receives a subsidy. If there are no subsidies, there are also no employer penalties.

4: Consumers could lose subsidies almost immediately.

Supreme Court decisions generally take effect 25 days after they are issued. That could mean that subsidies would stop flowing as soon as July or August, assuming a decision in late June. Insurers can’t drop people for non-payment of their premiums for 90 days, although they have to continue to pay claims only for the first 30.

Although the law’s requirement that individuals have health insurance would remain in effect, no one is required to purchase coverage if the lowest-priced plan in their area costs more than eight percent of their income. So without the subsidies, and with projected premium increases, many if not most people would become exempt.

5: Congress could make the entire issue go away by passing a one-page bill. But it won’t.

All Congress would have to do to restore the subsidies is pass a bill striking the line about subsidies being available through exchanges “established by the state.” But given how many Republicans oppose the law, leaders have already said to fix it. Republicans are still working to come up with a contingency plan should the ruling go against the subsidies. Even that will be difficult given their over health care.

States could solve the problem by setting up their own exchanges, but that is a and in most cases requires the consent of state legislatures. And the Obama administration has no power to step in and fix things either, Health and Human Services Secretary Sylvia Burwell said in a to members of Congress.