U.S. District Court Judge Reed O'Connor in Texas had ruled the law, known as the Affordable Care Act, was no longer constitutional because Congress had repealed the requirement that people buy insurance or pay a penalty.
Then he rattled off a number of deadly diseases - the kind of ailments that get coverage even if Americans hold preexisting conditions when they get insurance - one of the most popular elements of President Obama's health law.
That includes some 12 million low-income people covered through its Medicaid expansion, and some 11 million purchasing subsidized private health insurance through HealthCare.gov and state-run insurance markets.
The Justice Department is expected to elaborate on its position in a brief to the Appeals Court.
The Justice Department said it now agrees with the plaintiffs in a lawsuit challenging the law - a group of Republican attorneys general and governors. "Which means the Trump administration has now committed itself to a legal position that would inflict untold damage on the American public", wrote University of MI law professor Nicholas Bagley in a blog post.
The Trump administration is going to court to fight to dismantle the Affordable Care Act entirely.
Most legal experts view the case for ruling the entire law unconstitutional as thin.
It also would block the Trump administration from loosening "Obamacare" rules through waivers that allow states to undermine protections for people with pre-existing medical conditions or to scale back so-called "essential" benefits like coverage for mental health and addiction treatment.
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. However, during the 2017 congressional debate over repealing the health law, the nonpartisan Congressional Budget Office estimated the GOP replacement plans would result in steep coverage losses. John McCain of Arizona that saved the law.
Meanwhile, the DOJ is defending its decision.
Opposition Democrats, who have seen the law survive previous legal and legislative attacks, view the Affordable Care Act as a signature achievement of former president Barack Obama.
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".
About 11.8 million consumers nationwide enrolled in 2018 Obamacare exchange plans, according to the USA government's Centers for Medicare and Medicaid Services.