The Department of Justice (DOJ) on Monday announced that it is siding with a district court ruling that found the Affordable Care Act unconstitutional.
The move is an escalation of the Trump administration's legal battle against the health care law.
The DOJ previously argued in court that the law's pre-existing condition protections should be struck down. Now, the administration argues the entire law should be invalidated.
U.S. District Judge Reed O'Connor ruled in December that the Affordable Care Act's individual mandate is unconstitutional and that the rest of law is therefore invalid.
The DOJ said Monday that it agrees the decision should stand as the case works its way through the appeals process in the U.S. Court of Appeals for the 5th Circuit.
"The Department of Justice has determined that the district court’s judgment should be affirmed," the department said in a short letter to the appeals court.
The move is certain to prompt new denunciations from Democrats, who had already seized on the Trump administration's earlier call for the pre-existing condition protections to be struck down.
That stance was a major issue in last year's midterm elections, and many Republican candidates in tough races struggled with whether to say they agreed with the Trump administration's position.
Many legal experts in both parties think the lawsuit, which was brought by 20 GOP-led states, will not ultimately succeed. The district judge who ruled against the law in December is known as a staunch conservative.
The case centers on the argument that since Congress repealed the tax penalty in the law's mandate for everyone to have insurance in 2017, the mandate can no longer be ruled constitutional under Congress's power to tax. The challengers then argue that all of ObamaCare should be invalidated because the mandate is unconstitutional.
Most legal experts say legal precedent shows that even if the mandate is ruled unconstitutional, the rest of ObamaCare should remain unharmed, as that is what Congress voted to do in the 2017 tax law that repealed the mandate's penalty.
Justin Wise contributed to this report, which was updated on March 26 at 5:50 a.m.