U.S. District Judge Reed O'Connor in Texas ruled Friday that a change in tax law past year eliminating a penalty for not having health insurance invalidated the entire ACA.
The White House applauded O'Connor's ruling, but said the law remains in place while appeals proceed.
President Trump immediately tweeted following the ruling, urging Congress to pass a "strong" health care bill that would cover pre-existing conditions - a component that Democrats have long criticized Republicans for ignoring.
Trump lost no time in tweeting his delight at the court's ruling on a complaint brought by several Republican attorneys general and two Republican governors.
A federal judge in Texas ruled on Friday the entire Affordable Care Act is unconstitutional due to a recent change in federal tax law. They trace their argument to the Supreme Court's 2012 ruling in which Chief Justice John Roberts Jr. wrote for the majority that the penalty the law created for Americans who do not carry health insurance is constitutional because Congress "does have the power to impose a tax on those without health insurance".
Jost noted that in the 2012 case in which the Supreme Court upheld the ACA, a lower appeals court had found the individual mandate unlawful, but ruled it could be severed from the rest of the law. United States, concluding that the individual mandate is unconstitutional and that, as a outcome, the entire Affordable Care Act is invalid. "If they can't get it done in Congress, they'll keep trying in the courts, even when it puts people's pre-existing conditions coverage at risk", Obama said.
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His decision is nearly certain to be appealed all the way to the Supreme Court.
Congress, however, eliminated the tax penalty last December as part of President Trump's major tax overhaul. Republicans made multiple unsuccessful attempts to "repeal and replace" the Affordable Care Act in 2017, but many ran their midterm election campaigns this year promising protections for people with pre-existing conditions - a central piece of the Affordable Care Act.
The court ruling came on the eve of the deadline for people to sign up for ACA coverage for 2019. This resolution would allow the House general counsel to defend the health care law on the chamber's behalf.
About 20 million people have gained health insurance coverage since the law passed in 2010 without a single Republican vote.
"It's a big ruling, it's a great ruling for our country", said Trump.
The Kaiser Family Foundation, a nonpartisan research organisation, estimates that 52 million adults from 18 to 64, or 27 per cent of that population, would be rejected for coverage under the practices that were in effect in most states before the Affordable Care Act. The mandate requires almost all Americans to get health insurance or pay a penalty.
"They found one judge to agree, and now Medicaid expansion could be gone, pre-existing condition protections could be wiped out, prices skyrocket and millions lose insurance".