The Supreme Court has completed its first day of arguments on President Barack Obama's health care overhaul, a landmark case that could affect the lives of nearly every American.

During the nearly 90-minute hearing Monday, the justices' intensive questioning indicated that a legal issue with the potential to derail the case will not stand in the way of the proceeding.

The central issue in this case is a provision in the law requiring nearly all Americans to buy health insurance.

Supporters of the measure say the provision is needed to spread the cost of health care among all Americans. Opponents are arguing the measure violates the Constitution and tramples on individual liberties.

Under the new law, Americans who choose not to purchase health insurance will be required to pay a penalty. The question posed Monday was whether the penalty is a tax. A U.S. law prevents tax cases from being heard before the tax is paid.

Mr. Obama signed the law in 2010, but key portions will not be implemented until 2014.

The court's nine justices will hear six hours of oral arguments in the course of three days, the most the court has scheduled for a single issue since the 1960s.

The court is expected to issue its decision in June.

Scores of protesters in favor of the legislation waved signs, sang and chanted “Support Obamacare” outside the Supreme Court. A VOA reporter says fewer against the measure gathered, but challenged their opponents to impromptu, civil debates.

The law has also become an issue of debate on the campaign trail. Republican candidates are vowing to repeal the health care policy if they unseat President Obama in the November election.

Republican presidential candidate Rick Santorum showed up on the steps of the highest court Monday to voice his disapproval. Santorum used the opportunity to tout himself as the candidate most able to repeal the measure.

“And there's one candidate who is uniquely disqualified to make the case. It's the reason I am here, and he's not. The reason I talk about Obamacare and its impact on the economy and fundamental freedoms, and Mitt Romney doesn't. It's because he can't because he supported government run health care as governor of Massachusetts.”

Republican presidential candidate Mitt Romney supported a law requiring residents of Massachusetts to obtain health insurance while serving as governor.

The law — derisively labeled “Obamacare” — seeks to extend medical insurance to millions of Americans who do not have any. It has become a rallying point for conservatives who claim the changes will lead to bureaucrats replacing doctors in medical decision-making, and that the quality of health care will diminish.

Opponents of the requirement to buy health insurance, called the individual mandate, say Congress lacks the authority to force Americans to purchase the coverage. In briefs filed to the court, they say the mandate is “unprecedented” and a decision upholding it would do “irreparable damage” to the constitutional system.

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

The health care law also 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.

Proponents of the individual mandate say it will expand the market of healthy people who otherwise would not purchase insurance, helping offset the cost of covering people with higher medical bills.

This case comes before a divided bench made up of five justices appointed by Republican presidents and the rest appointed by Democrats.

The health care overhaul was the most significant reform to the U.S. health care system in four decades and a key portion of Mr. Obama's domestic agenda.