A US Supreme Court justice seen as a key swing vote challenged the landmark health reform Tuesday in a second day of arguments focusing on whether it is constitutional to require Americans to buy health insurance. Justice Anthony Kennedy\'s skepticism as he probed Solicitor General Donald Verrilli\'s defense of the Affordable Care Act raised doubts about the fate of President Barack Obama\'s major legislative achievement. But the court\'s conservative Chief Justice John Roberts appeared ambivalent, in a nine-member court that otherwise seemed divided along ideological lines that favor conservatives. Kennedy said the justices \"must presume laws are constitutional,\" but warned Verrilli he had \"a heavy burden of justification.\" Drawing a distinction from other cases involving the Congress\' right to regulate interstate commerce -- in this case insurance -- Kennedy said, \"Here the government says the individual has a duty, he must act -- and that\'s different.\" The exchanges came on the second of three unprecedented days of oral arguments in what is perhaps the court\'s biggest case since it had to decide on the Al Gore versus George W. Bush 2000 election, one with huge implications for the nation and the 2012 elections. Although Roberts took a milder approach in questioning Verrilli, he also raised concerns of \"a slippery slope\" if the law is upheld as other conservative justices seemed to side with the view that the state was compelling commerce in requiring all individuals to buy health insurance.  More liberal justices insisted that health care was different, and not like buying a car or broccoli. \"We all suffer from the cost of being sick,\" said Justice Steven Breyer. But Justice Antonin Scalia, a conservative, argued that \"everybody has to buy food, therefore you can make people to buy broccoli.\" He also questioned what powers would be left to the states if the federal government were able to mandate a requirement to buy insurance. \"What is left if the government can do this?\" he asked. The justices debated the law for two hours before a packed court that included Attorney General Eric Holder and a host of political luminaries from both sides of the aisle. Verrilli defended the administration\'s position that the reforms, which expand health care coverage to 32 million Americans who are currently uninsured, fall within the rights of Congress. Paul Clement, a former solicitor general arguing for 26 states challenging the law, claimed that the so-called \"individual mandate,\" requiring the purchase of insurance, is an assault on personal freedoms guaranteed under the constitution. As the session got underway, protesters for and against the law held demonstrations on the steps of the court, reflecting the intense passions the law has aroused in an election year. Tuesday\'s session followed oral arguments Monday in which the justices appeared to satisfy themselves they had jurisdiction to review the law even though it does not enter into full effect until 2014. On Wednesday, they will take up two other issues separately -- whether the law as a whole can survive if the \"individual mandate\" is found to be unconstitutional and whether the government can require the states to extend Medicare -- a federal program for low-income Americans -- to a larger pool. The individual mandate is a linchpin because it requires that all Americans maintain minimum insurance coverage from 2014, or pay a fine. Supporters hail the law -- the most sweeping reform of the troubled US health care system in decades -- as a major social advance, while opponents view it as an assault on individual liberties, deriding it as \"Obamacare.\" The area around the court remained a focus for protests on both sides, with several hundred people outside for a second day, some chanting, praying or singing. Twenty-three-year-old Abby Hoffman of North Carolina said she was there because \"health care is a human right.\" But on the other side, acknowledged Tea Party loyalist Diana Reimer of Pennsylvania said the government should not be allowed to order people to buy health insurance. \"I was raised to deal with my own problems, solve my own problems, and never did I go to the government to solve my problems,\" she said.