The conservative justices on the U.S. Supreme Court have questioned a key provision of the U.S. health care reform law, raising doubts about whether the government can require Americans to purchase insurance.

The oral arguments Tuesday were the second of three days on President Barack Obama's signature domestic policy, the Affordable Care Act.

Justice Antonin Scalia, one of the court's leading conservative thinkers, asked that if the law can require Americans to purchase a product, then, in his words, “What is left? If the government can do this, what else can it not do?”

But one of the four more liberal members of the nine-member court, Justice Stephen Breyer, said the issue shows that there is a “national problem that involves money, cost and insurance.”

The case before the nation's highest court represents a major legal and political showdown over the reform the president signed into law two years ago over the bitter objections of opposition Republicans.

Under the law, nearly every American will be required to buy a health insurance plan beginning in January 2014, or face a financial penalty. Supporters of the provision say it is needed to spread the cost of health care among all Americans, especially healthy people who might otherwise not purchase insurance, to cover more than 30 million uninsured people.

But 26 states and a coalition representing small businesses argue that Congress lacks the authority to force Americans to buy health insurance. Many businesses will also be fined if they do not offer health insurance to their employees.

The Obama administration says Congress does have the authority under its Constitutional powers to regulate interstate commerce and levy taxes.

Both supporters and opponents to the health care reform rallied outside the court Tuesday, while at the Capitol building, Republican senators voiced opposition to the law and Democratic legislators reiterated their backing of it.

The three days of oral arguments on the Affordable Care Act are the most the court has scheduled for a single issue since the 1960s.

The court spent Monday focusing on the technical question of whether a challenge to the law can be presented to the court prior to the law being fully implemented. Mr. Obama signed the bill in 2010, but key provisions do not begin until 2014.

The law is the most significant reform to the U.S. health care system in four decades. It bars insurance companies from denying coverage to people with pre-existing conditions or placing a cap on the benefits available to those with serious medical conditions.

The case comes before a divided bench made up of five justices appointed by Republican presidents and four appointed by Democrats.

The court is expected to issue its decision in June.