The White House has asked the Supreme Court to uphold its healthcare law, inviting a possible high-stakes legal showdown just before the 2012 election.
The administration's move came after a challenge to the reform from 26 states and small businesses.
The justice department asked the Supreme Court to declare the law's key provision, requiring everyone to buy health insurance, constitutional.
The legislation extended health coverage to an extra 32 million people.
Passed in 2010, the Affordable Care Act was a long-held dream of Democrats.
The White House's move on Wednesday followed an August decision by the 11th Circuit appeals court, in Atlanta, that the individual insurance mandate exceeded Congress' powers.
While the court said the remainder of the law was constitutional, it struck down its core requirement that Americans who can afford it must buy health insurance or pay a penalty from 2014.
The case was brought by 26 Republican attorneys general and governors, and small business owners, who oppose a provision forcing them to cover their employees' healthcare at a level set by the government.
The justice department said in a statement on Wednesday: "Throughout history, there have been similar challenges to other landmark legislation such as the Social Security Act, the Civil Rights Act and the Voting Rights Act, and all of those challenges failed.
"We believe the challenges to the Affordable Care Act - like the one in the 11th Circuit - will also ultimately fail and that the Supreme Court will uphold the law."
The healthcare law's Republican opponents want to repeal it in the courts, contending that the government cannot force people to buy health insurance.