The Trump administration has asked the US Supreme Court to invalidate Obamacare, which has provided health insurance to millions of Americans.
Government lawyers said the act became invalid when the previous Republican-led Congress axed parts of it.
Democratic challenger Joe Biden attacked the move, saying Mr Trump had put millions of lives at risk during the coronavirus pandemic.
Health care will be a key battleground in the November presidential election.
Some 20 million Americans could lose their health coverage if the court overturns the Affordable Care Act (ACA), which was introduced by Donald Trump’s Democratic predecessor, Barack Obama.
The act’s popular provisions include banning insurers from denying coverage due to pre-existing conditions and allowing children to stay on their parents’ health plans until age 26. Millions of low-income Americans were able to obtain insurance due to the act.
Mr Trump says the scheme costs too much and has promised a different plan to replace it, preserving some popular elements of the existing law but covering fewer people.
Under the act, millions of people in the United States must purchase health insurance or face a tax penalty.
But in 2017, Congress removed a key plank of the policy, eliminating the federal fine for those who did not sign up, known as the “individual mandate”.