They rely on a list of Supreme Court precedents that stretch the definition of “interstate commerce” pretty far.
In the 1940s, the court allowed Congress to punish a farmer for growing wheat on his own land for his own use, on the theory that wheat prices would be affected if everyone did that. In the 1960s, the court classified civil rights laws as “regulations of commerce” even when they involved businesses that did practically no interstate business. And in 2005, the court ruled that Congress could prohibit someone from growing marijuana in her yard for her personal medical use, because federal laws against drugs are a kind of economic regulation.
Still, the court has never held that the federal government may compel people to participate in commerce. And this is what makes the individual mandate unprecedented: Never before has Congress presumed to order average Americans to purchase a good or a service in the marketplace.
Simply from the standpoint of semantics, the law’s defenders face a challenge. As ordinarily understood, the word, “regulate,” implies rules for activity that people have freely chosen to engage in (running a business, for instance). The word doesn’t imply forcing people, say, to start a business in the first place.
Likewise, “commerce” implies economic activity – but someone who fails to buy health insurance is not engaged in economic activity.
Beyond these disputes over definitions lies a fundamental question about the extent of federal power: If Congress can force us to buy health insurance, what can’t it order us to buy?