On the first of three days of arguments -- the longest in decades -- none of the justices appeared to embrace the contention that it was too soon for a decision.
Outside the packed courtroom, marching and singing demonstrators on both sides -- including doctors in white coats, a Republican presidential candidate and even a brass quartet -- voiced their eagerness for the court to either uphold or throw out the largest expansion in the nation's social safety net since Medicare was enacted in 1965.
Tuesday's arguments will focus on the heart of the case, the provision that aims to extend medical insurance to 30 million more Americans by requiring everyone to carry insurance or pay a penalty.
A decision is expected by late June as Obama fights for re-election. All of his Republican challengers oppose the law and promise its repeal if the high court hasn't struck it down in the meantime.
On Monday, the justices took on the question of whether an obscure tax law could derail the case.
Audio of the day's argument can be found at: http://bit.ly/GSXEpf.
The 19th century law bars tax disputes from being heard in the courts before the taxes have been paid.
Under the new health care law, Americans who don't purchase health insurance would have to report that omission on their tax returns for 2014 and would pay a penalty along with federal income tax on returns due by April 2015. Among the issues facing the court is whether that penalty is a tax.
Solicitor General Donald Verrilli Jr., defending the health law, urged the court to focus on what he called "the issues of great moment" at the heart of the case. The 26 states and a small business group challenging the law also want the court to go ahead and decide on its constitutionality without delay.
But one lower court that heard the case, the federal appeals court in Richmond, Va., has said the challenge is premature. No justice seemed likely to buy that argument Monday.
The justices fired two dozen questions in less than a half hour at Washington attorney Robert Long, who was defending the appeals court ruling.
"What is the parade of horribles?" asked Justice Sonia Sotomayor, if the court were to decide the penalties were not a tax and the health care case went forward? Long suggested that could encourage more challenges to the long-standing system in which the general rule is that taxpayers must pay a disputed tax before they can go to court.
The questions came so quickly at times that the justices interrupted each other. At one point, Justices Ruth Bader Ginsburg, Elena Kagan and Sotomayor started speaking at the same time. Chief Justice John Roberts, acting as traffic cop, signaled Ginsburg to go first, perhaps in a nod to her seniority. Only Justice Clarence Thomas, as is his custom, stayed out of the fray.
Verrilli also faced pointed questioning over the administration's differing explanations for whether the penalty is a tax.
"General Verrilli, today you are arguing that the penalty is not a tax. Tomorrow you are going to be back and you will be arguing that the penalty is a tax," Justice Samuel Alito said.
Verrilli said Monday's argument dealt with the meaning of the word in the context of the 19th century law, the Anti-Injunction Act. Tuesday's session will explore Congress' power to impose the insurance requirement and penalty. In that setting, he said, Congress has the authority under the Constitution "to lay and collect taxes," including the penalty for not having insurance.
Still, he had trouble keeping his terms straight. Answering a question from Kagan, Verrilli said, "If they pay the tax, then they are in compliance with the law."
Justice Stephen Breyer jumped in: "Why do you keep saying tax?" Breyer reminded Verrilli he should be saying penalty.
"Right. That's right," Verrilli said.
The administration officials involved with the defense and implementation of the health care law, Attorney General Eric Holder, Health and Human Services Secretary Kathleen Sebelius, were in the courtroom Monday. Republican Sen. Jeff Sessions of Alabama and Florida Attorney General Pam Bondi also were in the crowd that filled the courtroom's 400 seats.
Outside the court building, about 100 supporters of the law walked in a circle holding signs that read, "Protect my healthcare," and chanting, "Care for you, care for me, care for every family." A half-dozen opponents shouted, "We love the Constitution!"
Republican presidential candidate Rick Santorum was there, too, declaring anew that GOP front-runner Mitt Romney has no standing to challenge Obama on the law since Massachusetts passed a somewhat similar version when Romney was governor. Santorum said, "If you really want Obamacare repealed there's only one person who can make that happen."
A four-person student band from Howard University was part of the group favoring the law, playing New Orleans-style jazz tunes.
People hoping for a glimpse of the action had waited in line all weekend for the relatively few seats open to the public. The justices allotted the case six hours of argument time, the most since the mid-1960s.
The justices also will take up whether the rest of the law can remain in place if the insurance mandate falls and, separately, whether Congress lacked the power to expand the Medicaid program to cover 15 million low-income people who currently earn too much to qualify.
If upheld, the law will force dramatic changes in the way insurance companies do business, including forbidding them from denying coverage due to pre-existing medical conditions and limiting how much they can charge older people.
The law envisions that insurers will be able to accommodate older and sicker people without facing financial ruin because of its most disputed element, the requirement that Americans have insurance or pay a penalty.
By 2019, about 95 percent of the country will have health insurance if the law is allowed to take full effect, the Congressional Budget Office estimates.
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 insurance requirement is unconstitutional.