The Trump administration has asked the US Supreme Court to invalidate the law set up by Barack Obama that added millions of Americans to the country's healthcare safety net.
The Affordable Care Act (ACA), also known as Obamacare, became invalid after the previous, Republican-led, US Congress axed parts of it, government advocate Noel Francisco argued in a filing to the court.
"Nothing the 2017 Congress did demonstrates it would have intended the rest of the ACA to continue to operate in the absence of these three integral provisions," Mr Francisco said late on Thursday.
The Trump administration signalled earlier in the day it would ask the court to declare the ACA illegal, setting up what is likely to be a key political battleground in this year's presidential election.
The Democratic House of Representatives Speaker Nancy Pelosi earlier described the move as "unfathomable cruelty" amid the coronavirus outbreak, which has hit the United States harder than any other country.
It was the ACA that has prohibited health insurers from denying coverage to Americans with pre-existing health conditions.
Donald Trump has criticised healthcare costs and coverage under Obamacare and has been promising since his 2016 campaign to replace it with a different plan.
Democratic presidential candidate Joe Biden called the bid to overturn the law "cruel, heartless (and) callous".
Democrats expect to make their promise to defend Obamacare a marquee election-year issue after they used a similar strategy in 2018 to win control of the House of Representatives.
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