* Supreme Court could easily rule either way
* Four liberals likely to vote to uphold insurance mandate
* At issue whether Congress exceeded its power
By James Vicini
WASHINGTON, Aug 15 (Reuters) - The legal fate of President Barack Obama's signature healthcare law will likely come down to two Republican appointees on the U.S. Supreme Court -- Chief Justice John Roberts and Justice Anthony Kennedy.
That would be a familiar role for Kennedy, a moderate conservative who often has cast the decisive vote on the most contentious issues before the nine-member high court divided between conservative and liberal factions.
A Supreme Court ruling on the healthcare law, adopted by a Democratic-controlled Congress after a bruising political battle, could be a defining moment for Roberts, who was named chief justice in 2005 by Republican President George W. Bush.
"Given the deep ideological divisions over the case and the lack of precedent clearly on point, the court could easily rule either way," Ilya Somin, associate law professor at George Mason University, wrote in a recent blog post.
U.S. appeals courts have issued conflicting rulings on whether Congress exceeded its power under the Constitution when, in adopting the healthcare law in 2010, it required that Americans buy insurance or face a penalty.
The latest decision, handed down on Friday from an Atlanta-based appeals court, struck down that individual mandate provision, making it more likely the Supreme Court will get involved.
University of Richmond assistant law professor Kevin Walsh said the Supreme Court seemed virtually certain to decide the issue by the end of June next year. That would mean a ruling before the U.S. elections in November 2012, with the law seen as a major political issue.
Obama has championed the individual mandate as a major accomplishment of his presidency and as a way to try to slow the soaring costs of healthcare while expanding coverage to more than 30 million Americans without it.
"If the Supreme Court follows existing precedent, existing law, it should be upheld without a problem," Obama said in Minnesota during a town hall discussion. "If the Supreme Court does not follow existing law and precedent, then we'll have to manage that when it happens."
Republican presidential candidates have strongly criticized the law as costly and evidence of intrusive government power.
Legal experts said the court's four liberals, all appointees of Democratic presidents, were likely to uphold the individual mandate. Justices Stephen Breyer and Ruth Bader Ginsburg were appointed by President Bill Clinton while Obama named Justices Sonia Sotomayor and Elena Kagan.
The experts said Justice Clarence Thomas was expected to vote to strike down the mandate, based on his past opinions, and could be joined by fellow conservatives, Justices Antonin Scalia and Samuel Alito. All three were named by Republican presidents.
That would leave Roberts, a conservative who on occasion has refused to join far-right positions taken by Scalia and Thomas, and Kennedy to control the outcome.
Kevin Russell, a Washington lawyer who argues before the Supreme Court and who has followed the healthcare law, said Kennedy has been one of the justices most protective of state power against federal government encroachment.
"If he views the mandate as invading an area of traditional state authority, I think he may be one of the least likely justices to vote to uphold it," Russell said.
Orin Kerr, a George Washington University law professor, predicted Roberts and Kennedy both would likely end up voting to uphold the individual insurance mandate.
He cited an opinion by Kennedy in 1995 and the expansive view that Roberts recently supported of the power of Congress under the Constitution to adopt laws necessary and proper. (Additional reporting by Jeremy Pelofsky; editing by Howard Goller and Mohammad Zargham)