"I think the justices probably came into the argument with their minds made up. They had hundreds of briefs and months to study them," said Thomas Goldstein, publisher of SCOTUSblog.com and a prominent Washington attorney, though he conceded that "the oral arguments (in March) might have changed their minds around the margin."
Americans are largely split over the reform effort and its legality, according to polling.
A March poll for CNN by ORC International found that while support for the law appears to be growing, 50% of Americans opposed the law, 43% supported it and 7% had no opinion.
Nearly three-quarters of respondents said they wanted the Supreme Court to overturn at least some of the law's provisions, although the poll did not specify which ones.
The law, which helped spur the creation of the conservative tea party movement, is likely to be a centerpiece of the presidential election campaign.
Obama's presumptive Republican opponent, Mitt Romney, has promised to repeal the measure if elected.
But 76% of respondents in the March CNN/ORC poll said a Supreme Court ruling against the law still wouldn't change their minds about whom to vote for in November.
The 2010 passage of the Patient Protection and Affordable Care Act came after months of bare-knuckled fights over politics and policy and a century of federal efforts to offer universal health care.
The legislation signed by Obama reached 2,700 pages, nine major sections and 450-some provisions.
The first lawsuits challenging the health care overhaul began just hours after the president signed the legislation.