No matter how the court rules - a decision is expected by June 30 - the result seems sure to play a central role in this year's presidential election, and to profoundly influence public policy for decades.
"Whether one agrees or not, this is the most wide- ranging piece of federal legislation in a generation," said Temple University constitutional-law professor Robert Reinstein, former dean of the law school and a former Justice Department attorney. "Whenever the Supreme Court deals with the scope of federal power, it is a very important legal question."
He added, "I think it is huge."
The Patient Protection and Affordable Care Act, which won final passage March 21, 2010, with no Republican support, is Obama's signature legislative initiative. It seeks to transform the nation's health-care market by extending coverage to tens of millions of uninsured Americans.
Proponents say the law - which requires nonexempt individuals to carry insurance, raises a variety of taxes, and bans insurers from denying coverage based on a person's health condition - is the culmination of a century-long effort to see that the overwhelming majority of Americans have health care.
Opponents say it is an unwarranted intrusion on individual liberty.
Although the court is divided along ideological lines, there is no clear indication of how it will rule.
"To some extent, the question of the individual mandate is unprecedented," Carl Tobias, a constitutional-law professor at the University of Richmond law school, said of the law's requirement that citizens carry health insurance or pay a penalty. "I don't think it is as important as Brown [Brown v. Board of Education, the 1954 school desegregation case], but it may be the most important case of the past 50 years."
The case features an A-list cast of constitutional lawyers, from former U.S. Solicitor General Paul Clement, a hero to conservatives, who served under President George W. Bush and who is arguing against the law, to Donald Verrilli, the current solicitor general, and former corporate lawyer and Supreme Court clerk.
Interest groups as ideologically diverse as the National Federation of Independent Business, the American Civil Liberties Union, and the AFL-CIO have weighed in with briefs totaling thousands of pages. Competition for seats in the courtroom has been fierce.
Enactment of the law helped fuel the rise of the tea party movement, figured in the Republican takeover of the House in 2010, and has since become a rallying cry for Republican candidates vying to win the GOP presidential nomination. Polls generally reflect public opposition to the law, but advocates have argued that will change as it takes effect and voters see its advantages.
A requirement that parents be permitted to carry dependent children on their insurance policies through age 26 already is in place, along with free preventive care, no lifetime caps on medical coverage, and lower costs for seniors for prescription drugs.
The Supreme Court agreed in November to hear the case after federal appeals courts divided on its constitutionality. The court has scheduled an extraordinary six hours of oral argument over three days, the most time devoted to a case in 45 years.
In large part, the case hinges on a requirement that all nonexempt citizens purchase health insurance, or be forced to pay a penalty to the Internal Revenue Service.
What has riveted legal scholars is that never before has Congress required citizens to buy a product, or at least so directly.
The president and his allies say that provision is needed to create a pool of people large enough to extend coverage to about 32 million Americans currently without health coverage.
As its legal basis for requiring citizens to carry health insurance, the Obama administration cites the commerce clause of the Constitution, which gives the federal government broad power to "regulate commerce . . . among the several states."
Constitutional scholars say the clause was among the least controversial issues facing the Founders, and generated little debate during the drafting of the Constitution.
But since the 1940s, the law has helped drive a vast expansion of federal power, and provided the legal justification for everything from crop price supports to the 1964 Civil Rights Act to the federal ban on the cultivation of marijuana for home consumption.
The Supreme Court typically has been sympathetic.
"Since the New Deal, the court has been very deferential, and allowed [Congress] a very broad scope for exercising authority under the commerce clause," Tobias said.
Yet for many critics, the Affordable Care Act constitutes a breathtaking expansion of federal power. If the government can tell citizens to purchase insurance, why not General Motors cars? they ask.
"Upholding the individual mandate would basically endorse a broad version of federal power that has few legal limits," said Steven Engel, a trial and appellate lawyer with the University City-based law firm Dechert L.L.P., who wrote one of the amicus briefs arguing against the law.
But Kermit Roosevelt, a constitutional-law professor at the University of Pennsylvania law school, said the nature of the health-care system was so distinct that it was inconceivable the government would insert itself into other forms of commerce in the same way.
And were it to do so, it would do it far more directly, by simply using taxpayer money to assist one industry or another.
"It would be much more efficient to simply take your money and give it to General Motors, which in fact they did," he said.
Based on lower-court opinions, Roosevelt said he was reasonably confident the court would vote to uphold the mandate.
If the pattern holds, he said, the four Democratic appointees on the court will join with at least one Republican to affirm its constitutionality.
The legal tug-of-war over how far the Constitution will permit the government to go in regulating behavior centers on a 1942 Supreme Court decision in Wickard v. Filburn, in which the court upheld federal limits on wheat production.
To prop up Depression-era prices, the government had given Ohio farmer Roscoe Filburn, as well as other farmers, a quota on how much wheat they could grow.
Filburn said he had a right to grow wheat for personal consumption, but the government disagreed. It said the effect of tens of thousands of farmers growing wheat for their own use, including as feed for livestock on their farms, would be to undermine price supports.
The Supreme Court came down on the side of the government, and the result has been a steady expansion of federal authority asserted under the commerce clause.
The latest example was a 2005 decision by the Supreme Court in Gonzales v. Raich upholding a federal law banning the cultivation of marijuana for personal consumption.
Under a narrow reading of the Constitution, the states, not the federal government, would have jurisdiction in such cases, by exercising their police powers.
But the court held that under the commerce clause, the federal government had the authority, since large-scale home cultivation could conceivably create a market, and the government had the authority to regulate it.
Justice Antonin Scalia's concurring opinion in the Gonzales case has drawn particular attention. In it, he endorsed sweeping federal authority to regulate commerce, even if the activity did not cross state lines. To some scholars, Scalia's opinion suggests he might vote to uphold the individual mandate.
But it might not be that simple. Scalia also is known as a stickler for procedure. The court will also be looking at whether an obscure federal law barring lawsuits to overturn taxes before they are collected might be a basis for throwing out the challenges, effectively leaving the law untouched.
If the penalties contained in the law are deemed a tax, that could be a convenient basis for the court to overturn the challenges - and punt on a politically divisive issue in an election year, leaving the key issues for another day.
Contact Chris Mondics at 215-854-5957 or firstname.lastname@example.org.