WASHINGTON (AP) — A conservative-leaning panel of federal appellate judges raised concerns about President Barack Obama's health care overhaul Friday, but suggested the challenge to it may be premature.

The arguments at the U.S. Court of Appeals in Washington over a lawsuit against Obama's signature domestic legislative achievement focused on whether Congress overstepped its authority in requiring people to buy health insurance or pay a penalty on their taxes, beginning in 2014.

But Judge Brett Kavanaugh (top), a former top aide to President George W. Bush who appointed him to the bench, said that he has a "major concern" that courts might not be able to rule on the law's constitutionality until 2015. That's because a federal law bars most challenges to tax-related legislation before the tax or penalty is paid.