The Trump administration told a federal court late Monday that it would ask judges to toss out the entire Affordable Care Act, a decision that cast further uncertainty over the future of a federal law that has extended health insurance to millions of Americans.

Justice Department attorneys said in a letter filed with the U.S. Court of Appeals for the Fifth Circuit in New Orleans that a lower court's ruling the health law is unconstitutional "should be affirmed" and that the "United States is not urging that any portion of the district court's judgement be reversed."

A coalition of Republican-led states brought a lawsuit, Texas v United States, arguing the entire health law should be tossed out.

In a ruling last December, U.S. District Court Judge Reed O’Connor in the Northern District of Texas agreed with the GOP-led states argument that the health law was unconstitutional. The judge ruled that the constitutional foundation for the law – the requirement that people buy insurance or pay a penalty – was no longer constitutional because Congress had repealed the penalty. O'Connor ruled that because that provision was so central to the health law, the whole thing had to be invalidated.

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The Trump administration had said it wouldn't defend the individual mandate in court, but Justice Department lawyers nonetheless had argued that some parts of the law, including an expansion of Medicaid that provides coverage for millions of people, could survive. The Justice Department's letter on Monday reversed that position, saying the administration now agrees with O'Connor that all of the law must go.

“The Justice Department’s indication that it will defend Judge O’Connor’s judgment in its entirety suggests it now believes the entire statute must fall, from coverage for pre-existing conditions to the Medicaid expansion, and everything in between," said Steve Vladeck, a law professor at the University of Texas. "That’s a major shift in position, and would have enormous consequences if ultimately accepted by the courts.”

Democrats have vowed to defend President Barack Obama's signature 2010 health law despite repeated GOP attempts to challenge the law in Congress and the courts.

A group of 17 Democratic attorneys general have filed documents to intervene in the case and defend the health law. The lower court's ruling is on hold pending a decision by the appeals court.

House Democrats also have held several hearings that highlighted health law provisions such as a requirement that prevents insurers from denying coverage based on existing medical conditions.