The Obama administration says a federal judge in Virginia was wrong when he struck down the centerpiece of the new health care law.
U.S. District Judge Henry Hudson ruled in December that it's unconstitutional to require people to buy health insurance or pay a penalty.
The administration said the Constitution does give the government the power to regulate economic activities between the states and that includes buying health insurance.
"As Congress found, the means of payment for services in the interstate health care market is economic activity that substantially affects interstate commerce," the Justice Department wrote. "The requirement that participants in the health care market have insurance to pay for the services they consume is thus a quintessential exercise of the commerce power."
Virginia Attorney General Ken Cuccinelli is expected to file a response brief in a month.
Both sides believe the U.S. Supreme Court will have the final say on the issue.
Oral arguments are set for May.