The reforms, a top priority of President Barack Obama, were approved by Congress in March after a fierce national debate. The Justice Department defended their legality late on Tuesday in the first response to a number of court challenges.
A conservative public interest group, the Thomas More Law Center, had filed a lawsuit in Michigan on March 23, the day Obama signed the law, and asked the court for an injunction to block it from taking effect.
The group said a provision requiring most Americans to buy health insurance under threat of financial penalty was beyond the scope of Congress' power and was an unconstitutional tax.
The group also said it violated their constitutional rights because federal tax dollars would be used to fund abortions.
Defending the law, the Justice Department said Congress acted to address a national problem, the minimum coverage provisions were constitutional and the lawsuit was premature because no one had been harmed by the law.
They bring this suit four years before the provision they challenge takes effect, demonstrate no current injury, and merely speculate whether the law will harm them once it is in force, the Justice Department told the court.
Enjoining it would thwart this reform and reignite the crisis that the elected branches of government acted to forestall, the administration said in a 46-page brief made available in Washington.
Several states have also filed lawsuits in Florida and Virginia challenging the law.
A lawyer for the Thomas More Law Center said he was not concerned by the issues raised by the administration. There were no surprises and we're prepared to respond to every argument they raise, said Robert Muise, senior trial counsel.
The Justice Department said that Congress did not exceed its authority. It said those who did not want to buy insurance may qualify for an exemption from any penalty and that U.S. law prohibits lawsuits aimed at blocking the collection of taxes.