Much of the terrain was easy going. Neither the justices nor the lawyers doubted that the healthcare and healthcare insurance markets involve interstate commerce — insurance and healthcare providers are usually national or at least regional operations, folks who cross state lines get sick and must be cared for away from home regularly, and people are often unable to relocate to another state for fear of losing employer-based coverage. Nor was it disputed that the mandate was sincerely motivated by and closely related to the regulation of these interstate markets. Those two conclusions are usually sufficient to justify the exercise of congressional power under the commerce clause of the Constitution.
But then things got more treacherous. The problem, suggested by numerous questions from the conservative justices on the court, was the slippery slope they saw created by the mandate — the idea that Congress was requiring individuals to buy something. If the feds can require each person to buy health insurance, what can’t they force people to purchase?