But a review of lower court rulings by conservative judges, subtle signals from individual justices, and interviews with professors and judges across the ideological spectrum suggest that presumption is wrong - and that the court will uphold the law.

Not that conservative court-watchers like to broadcast such a view in this combustible atmosphere.

"It's almost like they're confessing to some secret vice when they say they don't think (the law) should be struck down," said former U.S. Court of Appeals Judge Michael McConnell, a George W. Bush appointee who now teaches at Stanford Law School, referring to some fellow conservatives.

Several legal experts who do not necessarily favor the law, but bet it will survive, point to the decisions of two leading conservative federal appellate judges who already have sided with the Obama administration. The core of its healthcare law is a requirement that most people in the United States buy insurance by 2014.

In decisions upholding that so-called individual mandate last year, those judges stressed the classical conservative regard for judicial restraint and deference to Congress. While they wrote that the healthcare law might be flawed as a policy matter, they said decisions on how to reform the system were best left to legislators.

By contrast, three conservative judges who rejected the law took what some critics said was a more activist approach and said they were compelled to strike down the law because it exceeded congressional power. One invoked the 18th-century Boston Tea Party, in a decision widely viewed as a salute to the modern-day Tea Party movement's advocacy of less government involvement in people's lives.

In an interview, U.S. Appeals Court Judge J. Harvie Wilkinson, who was appointed to the 4th Circuit in Richmond, Virginia, by President Ronald Reagan, spoke of the philosophical division among Republican appointees to the federal bench.

"There's a real tension now in conservative ranks between those who want to use (the Constitution) to limit the scope and size of federal government," Wilkinson said, "and those of a more traditional strain of conservatism that believes that these epic battles should be left to the political branches to fight out."

Wilkinson, who was on George W. Bush's short list for the Supreme Court, has not presided over any of the challenges to the healthcare law that have made their way through U.S. courts.

But he suggested in a recent book on constitutional theory that the law was valid, subject to Congress' power to regulate commerce in the states: "The idea that Congress is constitutionally disabled under the commerce power from regulating activity affecting one-sixth of the national economy strikes me as a heavy lift." Healthcare spending totals more than 17 percent of U.S. gross domestic product.

TWO LIKELY CANDIDATES

If the four liberal justices vote to uphold the law, as legal analysts overwhelmingly predict, only one of the conservatives would be needed to make a majority. One likely candidate is Chief Justice John Roberts, who is generally seen as a more traditional conservative.

A 2005 appointee of George W. Bush, Roberts has often deferred to Congress. Most notably, he voted with liberals in a 2010 decision that endorsed congressional authority to require "sexually dangerous" prisoners to remain in state custody after completing their sentences. That decision in United States v. Comstock is among the cases the Obama administration relies on to defend the individual mandate.

Another conservative justice who could defy political-based assumptions is Anthony Kennedy. More than any current member of the court, he has straddled the middle and been the swing vote when the liberal wing has prevailed. That was seen as recently as Wednesday, when he joined the four liberals and wrote the decision extending the constitutional guarantee of effective legal assistance to defendants at the plea-bargain stage, not just at trial.

Kennedy's decisions relating to Congress' power to regulate interstate commerce - which will be key to the healthcare case - have been mixed. But in recent years, he has ruled for broader federal authority. In 2005, for example, Kennedy with four liberal justices - and conservative Antonin Scalia - declared that federal law trumped state policy allowing the cultivation of marijuana for medical use. That decision expansively interpreted federal commerce power and has become a major component of the Obama administration's legal defense of the healthcare law.

The Obama administration highlights that ruling, Gonzales v. Raich. In what some court watchers see as an attempt to speak to Kennedy, administration lawyers even quote him in a separate case where he voted to strike down the federal law at issue - banning guns at local schools - but stressed in his opinion that Congress had great regulatory power in national markets "to build a stable national economy."

CHALLENGING A MANDATE

At the heart of the healthcare law signed by Obama in March 2010 is the mandate that people buy insurance or face a tax penalty. That requirement is intended to bring more healthy people into the U.S. medical system, to share costs and offset the burden the uninsured put on the system when needing emergency care.

The challengers, 26 states and the National Federation of Independent Business, say such a mandate wrongly forces people who have opted out of the insurance market to spend money for a policy and, as a constitutional matter, exceeds Congress' power to regulate interstate commerce.

The Constitution gives Congress authority to regulate commerce "among the several states" and the Supreme Court has expansively interpreted that over the years to cover a range of local activities that affect a broader national market.

After the Supreme Court agreed to take up the healthcare litigation last November, many political commentators predicted the law was doomed, based partly on the makeup of the bench.

The nine justices often divide along ideological and even partisan lines, particularly in cases involving social policy questions such as racial discrimination, abortion rights and campaign finance. On the conservative side are five Republican appointees, Roberts and Justices Kennedy, Scalia, Clarence Thomas and Samuel Alito. On the liberal wing are four Democratic appointees, Justices Ruth Bader Ginsburg, Stephen Breyer, Sonia Sotomayor and Elena Kagan.

Unlike the 5-4 social policy cases, the healthcare dispute involves a structural question of the Constitution's grant of federal power.

The decisions of the two lower court judges who sided with the administration could provide a roadmap. Both Jeffrey Sutton, of the Cincinnati-based 6th U.S. Circuit Court of Appeals, Circuit, and Laurence Silberman, of the U.S. Court of Appeals for the Washington, D.C., Circuit, are highly respected conservatives who have worked closely in the past with current justices.

In separate rulings last year, Sutton and Silberman found that Congress' power to regulate interstate commerce could cover the individual insurance mandate. Most people will eventually enter or affect the insurance market, they observed.

Sutton, a former law clerk to Scalia and Ohio solicitor general appointed to the bench by President George W. Bush, wrote that "time assuredly will bring to light the policy strengths and weaknesses of using the individual mandate as part of this national legislation" and that any determination on whether it stands should be left to Americans' "political representatives, rather than their judges."

Silberman, a Reagan appointee who was a colleague to four of the current justices when they were on the D.C. Circuit, stressed that judges must "presume that acts of Congress are constitutional" and he cited opinions by Roberts and Kennedy in his decision upholding the individual mandate.

The conservative judges who voted against the law, on the other hand, were more ardent in their analysis and language.

In his decision invoking the Boston Tea Party, a forerunner to the American Revolution, U.S. District Judge Roger Vinson, based in Florida, wrote: "It is difficult to imagine that a nation which began, at least in part, as the result of opposition to a British mandate giving the East India Company a monopoly and imposing a nominal tax on all tea sold in America would have set out to create a government with the power to force people to buy tea in the first place."

U.S. District Judges Henry Hudson in Richmond, Virginia, and James Graham, sitting with the 6th Circuit appeals panel, called the healthcare law "legally stillborn."

AVOIDING CONTROVERSY

Other considerations suggest Roberts as chief justice might be inclined to uphold the law. He has spoken generally about the benefit of avoiding split decisions that could hurt the court's reputation and legacy in the American mind. In an interview with C-SPAN soon after he was appointed, Roberts said, "I think the most important thing for the public to understand is that we are not a political branch of government."

Roberts "seems sensitive to how he and the court are perceived," said Lawrence Baum, an Ohio State University political science professor who has studied judicial behavior.

Baum said Roberts and his fellow conservatives were likely aware of lingering criticism from the 2000 Bush v. Gore decision, ending the recounts in the Florida presidential election dispute, and in the 2010 Citizens United v. Federal Election Commission, lifting restrictions on corporate and labor money in elections.

"Given the decisions in which the court favored what are perceived as Republican interests," he said, "I think that at least some of the conservative justices welcome opportunities to appear to be above partisanship."

Several legal analysts also suggested that Roberts and Kennedy would seek to avoid a confrontation over a sitting president's signature domestic achievement. Law professors note that the Supreme Court has rarely voided a central plank of the president's agenda, the major exception being the early years of President Franklin Roosevelt's New Deal.

"This is not an ordinary law," said Akhil Amar, a law professor at Yale University who believes the healthcare law to be valid. This is a trademark piece of legislation."

(Reporting By Joan Biskupic; Editing by Amy Stevens and Peter Cooney)

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