The justice considered the most likely swing voter on the nine-member Supreme Court raised concerns Tuesday about the outcome of the 2010 US health reforms, during two hours of arguments in the landmark case, DPA reported.
Justice Anthony Kennedy has in recent years been the most likely of the court's five conservatives to join the four left-leaning justices in writing majority decisions.
The justices subjected lawyers for both sides to tough questioning, with Kennedy saying that the law's requirement for individuals to buy health insurance would change "the relationship between the individual and government in a fundamental way."
The four left-leaning justices seemed favourable to the law, considered President Barack Obama's most important domestic policy achievement, while only Chief Justice John Roberts among the five court conservatives voiced openness to the so-called individual mandate provision.