In a significant shift, the Justice Department now says it backs a full repeal of the Affordable Care Act, the signature Obama-era health law.

It divulged its position in a legal filing Monday with the U.S. Court of Appeals for the Fifth Circuit in New Orleans, where an appeal is pending in a case challenging the measure's constitutionality. A federal judge in Texas ruled in December that the law's individual mandate "can no longer be sustained as an exercise of Congress's tax power" and further found that the remaining portions of the law are invalid.

Previously, the Trump administration had not gone as far, arguing in a June brief that the penalty for not buying insurance could be distinguished from other provisions of the law, which could still stand.

Officials said there were legal grounds only to strike down the law's consumer protections, including those for people with pre-existing health conditions.

But in the new filing, signed by three Justice Department attorneys, the administration said that the decision of U.S. District Judge Reed O'Connor should be affirmed and that the entirety of the ACA should be invalidated.

"Because the United States is not urging that any portion of the district court's judgment be reversed, the government intends to file a brief on the appellees' schedule," the filing stated.

If it were successful, the Justice Department's position supporting the judge's ruling would potentially eliminate health care for millions of people and create widespread disruption across the U.S. health care system — from removing no-charge preventive services for older Americans on Medicare to voiding the expansion of Medicaid in most states. The change comes as newly empowered Democrats in the House have vowed to protect the ACA.

House Speaker Nancy Pelosi, D-Calif., pledged in a tweet Monday night that Democrats would "fight relentlessly" to preserve "affordable, dependable health care."

Timothy Jost, an emeritus professor at the Washington and Lee University law school, called the Justice Department's new position "crazy" and "legally untenable."

"I can't believe that even the Fifth Circuit would take that position," he said in an interview, suggesting that arguably the nation's most conservative appeals court would still be reluctant to accept the reasoning backed by the administration. "It would be like invalidating the Interstate Highway System, causing chaos on an unimaginable scale. It's conceivable that the entire Medicare payment system would collapse."

The filing reflected "a strictly political decision, not a legal decision," Jost said. "Trump has wanted to get rid of the ACA, and I guess he sees an opportunity here."

In the nine years since it was signed by President Barack Obama, the Affordable Care Act has become embedded in the nation's health care system. It changed the way Medicare pays doctors, hospitals and other health care providers. It has unleashed a wave of innovation in the delivery of health care. The health insurance industry has invented a new business model selling coverage to anyone who applies, regardless of any pre-existing conditions.

The law also includes dozens of provisions that are less well known and not related to the individual mandate. It requires nutrition labeling and calorie counts on menu items at chain restaurants. It requires certain employers to provide "reasonable break time" and a private space for nursing mothers to pump breast milk.

Lawyers said invalidation of the entire law would raise numerous legal and practical questions. It is, they said, difficult to imagine what the health care world would look like without the Affordable Care Act.

The Trump administration's new position was harshly criticized by the insurance industry and by consumer advocates.

The government's position "puts coverage at risk for more than 100 million Americans," said Matt Eyles, the president and chief executive of America's Health Insurance Plans.

Leslie Dach, the chairman of Protect Our Care, a consumer advocacy group, said: "In November, voters overwhelmingly rejected President Trump's health care repeal and sabotage agenda. But he remains dead set on accomplishing through the courts what he and his allies in Congress could not do legislatively: fully repeal the law, devastate American health care and leave millions of Americans at risk."

The New York Times contributed to this report.