Rather than offer a single compelling answer to this question, to date, the government has offered three.
In the 4th Circuit in Richmond, the government contended that Congress was limited to imposing mandates with regard to issues of truly national, as opposed to local, concern.
While this distinction is found in a 1942 case, it has never been adopted as a rule of law to be enforced by the courts. Nor will it be. Courts are never going to assess whether a problem is "truly" national or "truly" local, because they are incapable of drawing any such line in a principled fashion.
Instead they will defer, as they typically do, to the assessment of Congress that a particular problem is of national concern.
In the 6th Circuit last week in Cincinnati, the government offered a different limit: As the Supreme Court held in 1995, Congress may not regulate wholly intrastate noneconomic activity.
But this principle does not provide any meaningful limits on the use of economic mandates. If an economic mandate is allowed in this case because Congress has a "rational basis" for believing it is essential to its economic regulatory scheme, then mandates will forever be available whenever Congress enacts a scheme of economic regulation under its commerce power.
Under this reading of Congress' Commerce Clause power, therefore, Congress can mandate that every American buy broccoli or a gym membership if, in its opinion, such a mandate is essential to its regulation of the health care business.
To this argument, the government responds with its third purported limiting principle: Health care is "unique" from any other market because (a) the need for health care cannot always be predicted; (b) the expenses can be massive; (c) federal law requires emergency rooms to provide care to those who can't pay; so (d) costs are thereby shifted to the prices charged insurance companies, thereby raising insurance rates to everyone.
Moreover, (e) by allowing people to buy insurance on the way to the hospital, the ACA itself creates a massive new "free-rider problem" that only the individual mandate can solve.
Because it is purely a factual claim, however, "health care is unique" simply fails to provide a legal or constitutional limitation on the powers of Congress.
The Supreme Court has long abstained from second-guessing the factual judgments of Congress, it is not going to start now, and the government knows it.
The next time Congress decides to impose an economic mandate, the courts will defer to Congress' own assessment of whether another economic mandate is "essential."
Thus, if ACA is upheld here, it will be the last time such a mandate will ever be questioned in a court of law.
When the Constitution was proposed, its supporters denied that a bill of rights was necessary to protect the liberties of the people. They contended that the enumeration of powers in the Constitution was itself a bill of rights that would protect the liberties of the people.
Due to expansive judicial interpretations of the powers of Congress, however, we are a very long way from the scheme the founders enacted.
Today, only a categorical principle will preserve the protection of liberty afforded by the scheme of limited and enumerated federal power: Congress may not use its "power to regulate commerce . . . among the several states" to conscript the American people to do business with private companies.
Economic mandates are unconstitutional.
Randy Barnett teaches constitutional law at Georgetown Law Center. He also represents the National Federation of Independent Business in its challenge to the Affordable Care Act in the 11th Circuit U.S. Court of Appeals.