The last time there was such a confrontation over congressional power was in President Franklin D. Roosevelt's first term, when the court struck down a series of New Deal laws. It ruled, for example, that the government could not require employers to pay minimum wages or recognize unions.
In 1937, however, the court famously switched directions and backed off. A year later, the justices signaled they would look favorably on laws that regulate commerce, but would view more skeptically laws that infringe on individual or civil rights. That consensus has held since then, through both liberal and conservative eras.
At times, the court has drawn a line. It struck down a mostly symbolic federal law in 1995 that banned guns in school zones, and said gun possession did not involve a regulation of commerce.
But Justices Anthony M. Kennedy and Antonin Scalia said they agreed with the post-New Deal view that Congress had very broad power to regulate markets and commerce. They joined a 6-3 ruling in 2005 in a California medical marijuana case and said the federal authority to control the market in illegal drugs reached into the home of Angel Raich. She was growing marijuana for personal use to relieve her pain.
Scalia wrote that "Congress may regulate even non-economic local activity if [it] is a necessary part of a more general regulation of interstate commerce." Obama's lawyers cite Scalia's words to defend the mandate to buy health insurance. They say it is a "necessary part" of regulating the market in health insurance and guaranteeing coverage even to those who are seriously ill.
Most legal experts believed from the start — and still do — that the high court is likely to uphold the Affordable Care Act because of its long tradition of deferring to Congress on economic regulation.
The four Democratic appointees — Justices Ruth Bader Ginsburg, Stephen G. Breyer, Sonia Sotomayor and Elena Kagan — are almost certain to uphold the law.
Chief Justice Roberts is likely to play the leading role, befitting his position.
If Kennedy and Scalia shift away from the stance they took in the marijuana case, Roberts could join with them and the court's two other conservatives, Justices Clarence Thomas and Samuel A. Alito Jr., to strike down the law.
Or Roberts could join with Kennedy and possibly Scalia and the four Democrats to uphold the statute.
A third option is open as well. If the justices are split, they could opt to put off a ruling until after 2014, when the first taxpayers pay a penalty for their failure to buy insurance.
Yale law professor Akhil Amar says the justices should let the voters decide which side is right in November.
"They should say, 'If you don't like this, vote the bums out,' " he said.