Court: What's left of health law without mandate?

"If the government can do this, what else can it not do?" Justice Antonin Scalia asked. He and Justice Samuel Alito appeared likely to join with Justice Clarence Thomas, the only justice to ask no questions, to vote to strike down the key provision of the overhaul. The four Democratic appointees seemed ready to vote to uphold it.

Kennedy at one point said that allowing the government mandate would "change the relationship" between the government and U.S. citizens.

"Do you not have a heavy burden of justification to show authorization under the Constitution" for the individual mandate? asked Kennedy.

At another point, however, he also acknowledged the complexity of resolving the issue of paying for America's health care needs.

"I think it is true that if most questions in life are matters of degree ... the young person who is uninsured is uniquely proximately very close to affecting the rates of insurance and the costs of providing medical care in a way that is not true in other industries. That's my concern in the case," Kennedy said.

Justice Ruth Bader Ginsburg said she found the debate over health care similar to an earlier era's argument about the Social Security retirement system. How could Congress be able to compel younger workers to contribute to Social Security but be limited in its ability to address health care? she wondered.

"There's something very odd about that, that the government can take over the whole thing and we all say, Oh, yes, that's fine, but if the government wants to preserve private insurers, it can't do that," she said.

Associated Press writer Jesse J. Holland contributed to this report.