The Department of Justice said Thursday that it will not defend the constitutionality of key provisions of the Affordable Care Act in a lawsuit currently underway in Texas. Here’s what you need to know:

The background: Twenty states, led by Texas, sued the federal government in February as part of an effort to overturn the Affordable Care Act, arguing that, because the GOP tax bill eliminated the penalty for individuals who don’t buy health insurance, the law as a whole was unconstitutional. The Supreme Court’s 2012 ruling that upheld the Affordable Care Act called the individual mandate penalty a tax — and therefore legal. But the lawsuit argues that since the mandate can no longer raise any revenue, the mandate itself is now unconstitutional – and so is the entire ACA.

What they did: The Department of Justice filed a brief Thursday in support of the suit, saying “this Court should hold that the ACA’s individual mandate will be unconstitutional as of January 1, 2019,” and that some other provisions of the law – including those protecting people with pre-existing conditions being denied coverage or charged higher premiums – are inseparable from the mandate and therefore should be declared unconstitutional as well.

Why they did it: Republicans have targeted the Affordable Care Act for elimination ever since it passed, and President Trump continues to promise that it will be overturned. However, the federal support for the states’ lawsuit is unusual, since the Justice Department typically defends existing law. Attorney General Jeff Sessions wrote a letter to House Speaker Paul Ryan explaining that this is a “rare case” that warrants deviating from the Justice Department’s “longstanding tradition of defending the constitutionality of duly enacted statutes.” Tom Miller of the American Enterprise Institute told Politico that the refusal to defend key parts of the ACA shows that the Trump administration is going even further in its battle against the health care law.

What it means: If successful, the states’ lawsuit would allow insurers to charge much higher rates to people with pre-existing conditions, or deny them coverage altogether, effectively ending the ACA’s promise of providing health care for all Americans. If the Justice Department’s analysis is ultimately persuasive, however, other parts of the law, including Medicaid expansion, could stay in place. Some legal experts say the suit is weak, since it turns on the idea that if one part of a law is invalid, the whole thing is invalid, without recognizing that the Congress passed the law and is free to alter it while leaving the rest in place. But the lawsuit has been filed in a conservative court in Texas, and the Trump administration’s refusal to defend key parts of the law has likely boosted the plaintiffs’ chances.

Supporters of the health care law expressed considerable alarm on Friday. Andy Slavitt, who ran the Centers for Medicare and Medicaid Services under President Obama, tweeted that the Justice Department's decision is the “biggest health care news of the year” and a blow to public health. Slavitt and others criticized the move as an unprecedented decision by the Justice Department to not defend the rule of law. Ultimately, the case may be heading for the Supreme Court.