But then came United States v. Lopez and United States v. Morrison, two cases in which the court ruled that Congress had exceeded its authority under the Commerce Clause.
In both cases, Kennedy voted with the majority.
Lopez, decided in 1995, involved a law that made it a federal offense to possess a fire arm within 1,000 feet of a school zone.
The government argued that the possession of a firearm in a school zone could result in violent crime and that violent crime could be expected to affect the national economy. The court rejected the argument, ruling "The possession of a gun in a local school zone is in no sense an economic activity that might, through repetition elsewhere, substantially affect any sort of interstate commerce."
Kennedy voted with the majority but also issued a concurring opinion: "The statute makes the simple possession of a gun within 1,000 feet of the school a criminal offense. In a sense any conduct in this interdependent world of ours has an ultimate commercial origin or consequence, but we have not yet said the commerce power may reach so far," Kennedy wrote.
Siegel says Kennedy's concurrence reflects "that there are limits on the scope of Congress' commerce power, but they are modest limits. Congress complies with those limits as long as it is regulating the market."
The Morrison case involved a provision of the Violence Against Women Act that allowed victims of gender-motivated violence to sue in federal court. The court relied on much the same reasoning as in Lopez, and struck down the provision.
Randy Barnett, a professor at Georgetown University Law Center who is also representing a small-business group that is opposed to the health care law, says, "Justice Kennedy joined Chief Justice Rehnquist's 1995 opinion in Lopez -- a five-four decision -- holding that there were judicially enforceable limits on the power of Congress -- a proposition that had been in doubt for 60 years but which was reaffirmed in the 2000 case of Morrison."
In Lopez and Morrison, the court ruled that Congress had exceeded its authority. But supporters of the health care law point to another recent case in which the court upheld a law based on Congress' power under the Commerce Clause.
In Gonzales v. Raich, the court ruled that federal anti-drug laws could be applied to prohibit the local cultivation of marijuana for medical purposes, which had been authorized under state law.
The court held that even the homegrown cultivation and possession of the drug used solely to alleviate the chronic pain of people like Angel Raich would undercut Congress' regulation of interstate economic activity.
"Our case law firmly establishes Congress' power to regulate purely local activities that are part of an economic class of activities that have a substantial effect on interstate commerce." Justice John Paul Stevens wrote. Kennedy joined the opinion.
Dorf differentiates Kennedy's votes in Lopez and Raich, saying that Kennedy understood in Raich that marijuana was part of a national market.