The U.S. Supreme Court is holding another day of discussions on whether a key provision of President Barack Obama's health care reform, the individual mandate, is permitted under the U.S. Constitution.

Tuesday is the second of three days of arguments on Mr. Obama's signature domestic policy.

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.

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.

Both supporters and opponents to the law rallied outside the court Tuesday.

The nine-member 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 of the law do not begin until 2014.

The Affordable Care Act 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.