Today’s ruling by the 11^th Circuit Court of Appeals striking down the national health care law’s individual mandate hinged on an issue that has consistently tripped up the Obama administration during oral arguments in several of the legal challenges to the law. The essential question is: if courts uphold the individual mandate, what is the constitutional principle that would limit the U.S. Congress’s exercise of its Commerce Clause power?

This issue has often been framed by asking whether the power being claimed could allow future Congresses to force Americans to eat broccoli or join a gym. Obama’s lawyers, while acknowledging that there’s no Supreme Court case that directly grappled with the issue, have countered by making the “health care is unique” argument. That is, since virtually everybody will need health care at some point, it’s a special case. Yet as I wrote in June, “simply saying the health care market is unique doesn’t actually create a very clear or understandable limit to Congressional power.”