In a filing with a federal appeals court, the Justice Department said it agreed with the ruling of a federal judge in Texas that invalidated the Obama-era health care law.
"The Department of Justice has determined that the district court's judgment should be affirmed", the Monday night letter said.
Trump's Department of Justice (DOJ) had previously taken the position that it would not defend the law's protections for people with pre-existing conditions, though other parts could stay.
Meanwhile, millions of people continue to benefit from the ACA's taxpayer-subsidized private insurance plans, but enrolment is slowly declining and experts fear stagnation.
A coalition of Republican-led states filed a lawsuit in Texas a year ago against the federal government, arguing that Congress' decision to eliminate the Affordable Care Act's individual mandate penalty - a fee Americans paid if they didn't enroll in some form of health insurance coverage - should render the entire law invalid.
The suit in question begins with the 2017 tax law, which eliminated the health law's individual mandate but left the rest of it in place.
The legal argument driving the case remains weak: In post at the Volokh Conspiracy (which is hosted by Reason) Case Western University Law Professor Jonathan Adler, who helped develop an earlier court challenge to the health care law, writes that the decision is "astounding", and that it adopts "a highly strained and implausible approach to severability with little grounding or precedent". Upending the law could prove politically unsafe for the Trump Administration and Republicans who are backing the law's repeal.
In Washington, a capital city consumed with the political storm over special counsel Robert Mueller's Russian Federation report, Democrats are trying to show they also care about policy by falling back on an issue that worked well for them in last year's midterm elections. Pelosi has accused the Trump administration of jeopardizing the health care of millions with its attempt to scrap Obamacare through the courts.
"This lawsuit. threatens the healthcare of tens of millions of Americans across the country, from California to Kentucky and all the way to ME", said California attorney general Xavier Becerra, who has led the fight to preserve the law.
The US Supreme Court had previously upheld the penalty, which was created to avoid free-riding on the US healthcare system, as a constitutional exercise of the federal government's power to tax.
House Democrats, backed by Speaker Nancy Pelosi (D-CA), are slated to introduce new legislation to broaden and enhance the Affordable Care Act. "Millions of people would lose their health care coverage, and a host of seemingly unrelated policies - including new experiments in how Medicare pays for care and an entire class of prescription drugs - would also go out the window".
"Millions of Americans will be denied affordable health care", the House lawyers wrote. John McCain of Arizona that saved the law.
According to Pelosi's office, the new bill would increase aid for lower income participants, make more middle-class people eligible, and fix a "family glitch" problem which prevents certain consumers from joining. "Trump has wanted to get rid of the ACA, and I guess he sees an opportunity here". Unless younger, healthier people sign up, already-high premiums will march upward again.