The Supreme Court justices — five Republican appointees, four Democrats — opened three days of arguments into the law derisively dubbed "Obamacare" by the president's foes. The panel will make a ruling on the legislation in late June.
Their historic decision could deliver a humiliating defeat to Obama just five months before the presidential election, significantly rein in the reach of Congress and have a personal impact on virtually every American.
The battle over health-care reform has been red hot for the past three years in the U.S., the only major industrialized country in the world without a national health-care system until the law was passed.
Throughout much of the toxic debate leading up to the law's passage in March 2010, Canada's hallowed universal health care was alternately derided as an inefficient socialist nightmare and lauded as a Utopian model for a caring society.
On Monday morning, that battle raged on in front of the Supreme Court building as hundreds of supporters and opponents gathered.
Those in favour waved signs and chanted, "Protect our care" and "Don't deny my health care." Those opposed carried signs reading "Unlawful" as they chanted, "We love the Constitution."
"People of all ages are already receiving the benefits of the Affordable Care Act," Ron Pollack of Families USA told a news conference held by a coalition of organizations that support the legislation.
Behind him stood dozens of doctors and nurses.
Nearby, Tea Party protesters waved a yellow banner emblazoned with the movement's Don't Tread On Me slogan.
"If we get sick, they just want to get rid of us," Diana Reimer, a Tea Party Patriots organizer and senior citizen from Pennsylvania, told reporters.
The first question on the agenda for the nine Supreme Court justices was this: Was it premature for the court to be making any ruling on the legislation at all?
At the crux of that argument was an obscure 1867 law called the Anti-Injunction Act that says no tax can be opposed until it's been collected. The act would essentially prevent the Supreme Court from ruling on the health-care overhaul until Americans are actually forced to pay the penalty.
But at issue is whether the penalty Americans will have to pay in 2014, if they've failed to buy health insurance, is a tax.
The Justice Department says the so-called individual mandate requiring Americans to buy health insurance is constitutional, in part, because of Congress's power to levy taxes.
And yet in urging the court not to defer a decision until after 2014, Solicitor General Donald Verrilli Jr. argued that the penalty is not a tax.
"Tomorrow you're going to be back, and you're going to be arguing that the penalty is a tax," Justice Samuel Alito told him.
Neither the Obama administration lawyers nor those opposing the legislation have argued the Supreme Court has no business ruling now. But a lower court disagreed, and the Supreme Court asked to hear arguments on the Anti-Injunction Act.
The act "does not bar this court's consideration of the case," said Verrilli Jr., who's defending the health-care overhaul for the Obama administration, told the court amid 90 minutes of highly technical discussion.
The nine justices, conservative and liberal alike, posed questions that suggested they too were skeptical the 145-year-old act was at play.
Justice Sonia Sotomayor said she had come up with at least four similar cases in which the Supreme Court had allowed similar challenges despite the existence of the Anti-Injunction Act.
Obama had noble aims when he pushed for expansive health-care reform soon after his inauguration in early 2009.
The goal was to extend health coverage to more than 30 million uninsured Americans, to outlaw health insurers from discriminating against people with costly conditions and to prevent people from going bankrupt in the event they fell ill or suffered injuries while either uninsured or denied coverage.
But at the heart of the controversy is the requirement that most Americans buy health insurance or pay the penalty. Tuesday's hearing will be devoted to arguments for and against the individual mandate.
The requirement, similar to one that forces automobile owners to carry car insurance, has infuriated many in a country with a historical distrust of big government. Recent polls suggest a majority of Americans oppose the individual mandate.
Three federal judges stateside have already struck down the individual mandate as unconstitutional, while three others have upheld it.
Now the Supreme Court will decide whether to strike down or uphold the biggest expansion in America's social safety net since President Lyndon B. Johnson implemented Medicare and Medicaid in the 1960s. Among those challenging the law at the Supreme Court are 26 states, the National Federation of Independent Business and a handful of individuals.
Experts say there are already signs that the Affordable Care Act is saving money. A recent New England Journal of Medicine study says there has been slower growth in Medicare spending since the inception of the law, a decrease that was not tied to the economic recession.
Annual health-care spending in the U.S. totals US$2.6 trillion, about 18 per cent of the annual gross domestic product, or $8,402 per person. Canada spends just under $5,000 per person, a figure that would shave billions from America's national debt if the U.S. spent at a similar level.