The U.S. Supreme Court's conservative justices sharply questioned a core provision of President Barack Obama's historic health care reforms today, raising doubts about the fate of the requirement that all Americans must buy insurance coverage or face a penalty.

The court's ruling on the constitutionality of the law — expected in June, during the heat of the presidential re-election campaign — will affect the lives of nearly every American and stands to deepen already wide ideological divisions.

The Obama plan would extend medical insurance to 30 million Americans who go without coverage, either out of choice or an inability to pay the fast-rising premiums in the insurance private marketplace. Obama signed the measure into law two years ago, and it has since been challenged on constitutional grounds by 26 U.S. states and a business organization.

Before the Obama health reforms were signed into law, the United States was the only developed country without a national health care program.

The conservative justices pressed the government's lawyer about the right of Congress to force Americans to buy a product.

"If the government can do that, what else can it" do? asked Justice Antonin Scalia, referring to the individual mandate portion of the Patient Protection and Affordable Care Act.

Scalia, as well as Chief Justice John Roberts and Justices Samuel Alito and Anthony Kennedy, pressed Solicitor General Donald Verrilli on whether people can be forced to buy things like cars and broccoli if the government can make them buy health insurance.

Mandate would 'change the relationship'

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

"Do you not have a heavy burden of justification to show authority under the Constitution" for the individual mandate? asked Kennedy, who is often the swing vote on cases that divide the justices along ideological lines.

Scalia repeatedly pointed out that the federal government's powers are limited by the Constitution, with the rest left to the states and the people. "The argument there is that the people were left to decide whether to buy health insurance," Scalia said.

The Democrat-appointed liberal minority of four justices was expected to vote to uphold the Obama plan. But a ruling in favour of the law would require at least one of the five conservative, Republican-appointed judges to break ranks.

The health care overhaul, which squeaked through Congress when Democrats still controlled both houses, is constructed to expand the number of people who have insurance, including young and healthy people who may have fewer need for the system.

That is designed to offset losses to insurance companies, which would now be prevented from denying coverage to people who already have health problems — so-called pre-existing conditions. The plan also expands Medicaid, the federal-state government program that insures low-income and disabled Americans, with most of the additional cost borne by Washington.

At the close of the first of three days of hearings on Monday — the longest in decades — questioning by the nine justices had indicated they did not embrace the contention that a 19th century tax law would cause them to delay a decision on the constitutionality of the health care overhaul. The first day of arguments dealt with the 150-year-old law that holds courts cannot decide tax questions until the taxes are levied.

Penalty not effective until 2014

The penalty for not acquiring health insurance does not become effective until 2014 and would be collected along with federal income taxes that become due in April 2015. Both sides of the health care issue want a decision in this court session and argued the law does not apply.

The administration contends Congress has ample authority to impose the insurance mandate, arguing that health care costs consume 17 per cent of the American economy and are susceptible to the federal regulation of national commerce. Opponents demand the law be struck down as an unprecedented extension of federal power over individual liberties. They say that not even decades of high court rulings that endorsed an expansive view of congressional authority can support the health care law.

All of Obama's top Republican challengers oppose the law as an assault on economic freedoms and, should they win, promise its repeal if the high court hasn't struck it down by then.

Polls have consistently shown the public is at best ambivalent about the benefits of the health care law, and that a majority of Americans believe the requirement to buy insurance is unconstitutional.