WASHINGTON – The Obama administration’s top lawyer faced aggressive and often skeptical questioning by U.S. Supreme Court justices on March 27 in a sweeping and historic argument over the constitutionality of the nation’s new health care law.

At the end of fast-paced arguments in a courtroom packed with press, politicians and the public, the fate of the controversial so-called individual mandate to buy health insurance seemed to rest with two justices: Chief Justice John Roberts Jr. and Justice Anthony Kennedy.

The question before the justices during the two-hour argument was whether Congress exceeded its lawmaking power by requiring eligible individuals to purchase health insurance or pay a penalty for the failure to comply.

Read briefs submitted for Tuesday’s arguments and a transcript of the proceedings.

“Under the commerce clause, what Congress has done is to enact reforms of the insurance market, directed at the individual insurance market, that preclude discrimination based on pre-existing conditions, that require guaranteed issue and community rating, and the minimum-coverage provision is necessary to carry into execution those insurance reforms,” Solicitor General Donald Verrilli Jr. told the Court.

As Health and Human Services Secretary Kathleen Sebelius and senators, including Minority Leader Mitch McConnell, R-Kent., and Judiciary Chairman Patrick Leahy, D-Vt., looked on, Mr. Verrilli soon faced a barrage of questions in which various justices, primarily on the conservative side of the Court, pressed him on why the health insurance market was unique and what other products or services Congress could require citizens to buy.

To Mr. Verrilli’s claim that the distinguishing feature of the health care market is that people cannot control when they enter the market or what they need when they enter it, Justice Roberts countered that the same was true of police, fire, ambulance and even roadside assistance.

“So can the government require you to buy a cellphone because that would facilitate responding when you need emergency services?” Justice Roberts asked. “You can just dial 911 no matter where you are?”

The fundamental difference, Mr. Verrilli responded, is that is not an issue of market regulation, which is how Congress looked at the problems with the health insurance market.

Justice Samuel Alito Jr. asked about the difference between the health insurance market and the market for burial services, and Justice Antonin Scalia asked about the market for food. “Everybody has to buy food sooner or later, so you define the market as food, therefore, everybody is in the market; therefore, you can make people buy broccoli,” Justice Scalia said.

Before the arguments, Justice Kennedy was seen by many experts as the key vote in determining the outcome of the challenge to the Patient Protection and Affordable Care Act. During the arguments, he asked the question that has dogged the government almost from the beginning of the litigation.

“Can you identify for us some limits on the commerce clause?” he asked Mr. Verrilli. The solicitor general answered that Congress could not justify forced purchases of commodities to stimulate demand or purchases of insurance in situations where insurance does not serve as the method of payment for service.

“Here Congress is regulating existing commerce, economic activity that is already going on, people’s participation in the health care market, and is regulating to deal with existing effects of existing commerce,” Mr. Verrilli said.

He also told the justices that the challengers’ argument that this is a novel exercise of Congress’ commerce clause power is “ironic” because Congress chose to rely on market mechanisms and a method that has more choice involved than the traditional Medicare/Medicaid model.

After a brief break for a stretch, the justices opened the second hour of argument with former Bush Solicitor General Paul Clement of the Bancroft firm, counsel to 26 state attorneys general in their challenge to the law.

Mr. Clement told the justices that the commerce clause gives Congress the power to regulate existing commerce but not to compel people to enter commerce in the first place.

While Mr. Verrilli faced his toughest questioners in the Court’s conservative members, arguments by Mr. Clement and, later, Michael Carvin of Jones Day, who represents the National Federation of Independent Business, were vigorously probed by the Court’s more liberal justices.

In response to Justice Sonia Sotomayor, Mr. Clement said Congress could impose a mandate to purchase insurance at the point of consumption. Justice Elena Kagan interjected that Congress “surely has within its authority to decide, rather than at the point of sale, given an insurance-based mechanism, it makes sense to regulate it earlier. It’s just a matter of timing.”

But Mr. Clement said it was not a matter of timing alone. “If Congress tried to regulate at the point of sale, the one group that it wouldn’t capture at all are the people who don’t want to purchase health insurance and also have no plans of using health care services in the near term. I mean, those people are essentially the golden geese that pay for the entire lowering of the premium.”

But Justice Ruth Bader Ginsburg said the law’s premise works in the way that Social Security works. Congress in the 1930s saw a national problem of people needing old-age and survivor insurance, she said. It enacted a tax requiring everyone to contribute because without the healthy and young, she said, there would not be enough money to pay for those who became old, disabled or widowed. That tax was constitutional.

“It was a big fuss about that in the beginning because a lot of people said—maybe some people still do today—I could do much better if the government left me alone,” she said. “So if Congress could see this as a problem when we need to have a group that will subsidize the ones who are going to get the benefits, it seems to me you are saying the only way that could be done is if the government does it itself; it can’t involve the private market; it can’t involve the private insurers.”

Mr. Clement responded that Congress had alternative ways of dealing with the problem.

Mr. Carvin of Jones Day told the justices, “It is clear that the failure to buy health insurance doesn’t affect anyone. Defaulting on your payments to your health care provider does. Congress chose for whatever reason not to regulate the harmful activity of defaulting on your health care provider.” The “fundamental problem” with the individual mandate, he said, is that it makes young healthy people subsidize insurance premiums for the cost that the nondiscrimination provisions in the law have put on insurance premiums and insurance companies.

But Justice Kennedy said, “The young person who is uninsured is uniquely, proximately very close to affecting the rates of insurance and the costs of providing medical care in a way that is not true in other industries. That’s my concern in the case.”

The Court returns on Wednesday to consider two other questions about the new law. First, if the mandate is unconstitutional, can it be severed from the act or must the entire act fall? And second, does the expansion of Medicaid unconstitutionally coerce the states into compliance?