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

It presented its position in a legal filing Monday with the U.S. Court of Appeals for the 5th 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 void. He based his judgment on changes to the nation’s tax laws made by congressional Republicans in 2017.

At first, the Trump administration had not gone as far, arguing in a brief last June that the penalty for not buying insurance was legally distinct from other provisions of the law, which could still stand. Justice Department officials said there were grounds only to strike down the law’s consumer protections, including those for people with preexisting 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 the entirety of the ACA should be invalidated.

The government said it planned to file a brief in support of the Texas-led coalition of states pursuing the law’s complete nullification, now that “the United States is not urging that any portion of the district court’s judgment be reversed,” as the filing stated.

“The Department of Justice has determined that the district court’s comprehensive opinion came to the correct conclusion and will support it on appeal,” Kerri Kupec, a spokeswoman for the Justice Department, said in a statement.

If the Justice Department’s position prevails, it would potentially eliminate health care for millions of people and cause 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. A court victory would fulfill Republican promises to undo a prized domestic accomplishment of the previous administration but leave no substitute in place.

The Justice Department announced the shift fewer than six weeks after William P. Barr was sworn in as attorney general. Barr, who served in the same position under President George H.W. Bush, is currently at the center of a conflict over the findings reached by special counsel Robert S. Mueller III in his investigation of Russian interference in the 2016 election — a conflict about who is entitled to view those findings, and who has the authority to make judgments about them.

As that controversy raged, Monday’s filing — on the scarcely less explosive issue of health coverage — offered a window into Barr’s approach to his role as the nation’s top law enforcement officer, said Nicholas Bagley, a professor of law at the University of Michigan.

“This refutes any notion that people might have that Attorney General Barr would retreat from some of the most partisan litigation choices made by the previous attorney general,” Bagley said in an interview, referring to former attorney general Jeff Sessions, who declined to defend the ACA against legal challenges in a break from the executive branch’s long tradition of supporting existing statutes.

The new position staked out by the Justice Department, which fully abandons the health law, brings the administration in line with the consortium of Republican-controlled states arguing that no portion of the measure is constitutionally sound. The U.S. House and a California-led coalition of Democratic states have intervened to ask that the law be upheld in its entirety.

The change comes as newly empowered Democrats in the House have vowed to protect the ACA from GOP attacks. In midterm races last fall that restored their majority in that chamber, Democrats hammered their rivals for pursuing an eight-year crusade against the law, commonly known as Obamacare.

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

Meanwhile, there were already signs that the one-page memo could become a cudgel for Democratic candidates vying to wrest the White House from President Trump.

“Trump and his administration are trying to take health care away from tens of millions of Americans — again,” warned Sen. Kamala D. Harris of California, who is among the Democratic aspirants who have signaled support for a Medicare-for-all system. In 2020, she added, “we need to elect a president who will make health care a right.”

Legal experts said the filing was more significant in demonstrating the administration’s anti-ACA zeal than in altering the course of ongoing litigation.

Timothy S. Jost, an emeritus professor at Washington and Lee University School of Law, called the Justice Department’s new position “crazy” and “legally untenable.”

“I can’t believe that even the 5th 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. He thinks maybe he can get the 5th Circuit to go along and, ultimately, maybe the Supreme Court."

But the prospect of another airing at the nation’s top court, which has twice sustained the law, would fade if the 5th Circuit were to reverse the judgment reached in Texas, Bagley predicted. In that scenario, he would expect the Supreme Court to decline to hear the case on appeal.

The justices would be more likely to step in, he said, to undo a finding by the circuit court embracing the logic now backed by the administration.

“The sheer irresponsibility of the notion that you would rip the Affordable Care Act out of the American health-care system without having any prospect for a transition plan, or much less a replacement, is extraordinary,” Bagley said. “It’s breathtaking.”

Amy Goldstein contributed to this report.

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