On the other side, Justice Clarence Thomas has made clear in several cases that he does not take an expansive view of Congress' powers.
Both the Obama administration and the health-care law's challengers say they believe they can attract the four other Republicans to their side. The group consists of Chief Justice John G. Roberts Jr. and Justice Samuel A. Alito Jr., the two appointees of President George W. Bush who have swung the court to the right in a number of areas; conservative stalwart Antonin Scalia; and Anthony M. Kennedy.
The court has set aside six hours over three days, beginning next Monday, to hear arguments - the most time for any issue in more than 45 years.
Some supporters of the law worry about the court's decision because a similar partisan split, with a few important exceptions, has emerged in the lower courts.
Among lawyers who appear regularly before the court, Carter Phillips of Sidley Austin L.L.P. offered a fairly typical response in predicting the court is more likely to uphold the law than strike it down. But he was far from certain.
"I have no doubt that if the court strikes down the statute, it will be 5-4," he said. "But if the court upholds the statute, it would not shock me if the decision turned out not to be 5-4."
For months, Kennedy has been seen as the most likely Republican to vote in favor of the law. He often provides the decisive vote in cases that divide the court by ideology.
But his record in cases involving Congress' constitutional power to regulate commerce offers both sides some hope. Kennedy was part of the majority that struck down a federal law prohibiting people from carrying guns in or near schools.
At the same time, he said in a concurring opinion in the case that "Congress can regulate in the commercial sphere on the assumption that we have a single market and a unified purpose to build a stable national economy."
The Obama administration says Kennedy could have been describing health care, which makes up 17 percent of the U.S. economy.