When a reporter asked then-Speaker of the House Nancy Pelosi (D-Calif.) back in October 2009 "where specifically does the Constitution grant Congress the authority to enact an individual health insurance mandate?", Pelosi's response was to dismiss both the reporter and the question. "Are you serious?" she sneered. Nadeam Elshami, Pelosi's communications director, later amplified his boss's response, telling CNS News, "You can put this on the record. That is not a serious question."

The U.S. Supreme Court thinks that it is. On Monday March 26, the Supreme Court will begin hearing three days of oral arguments devoted to the constitutionality of the Patient Protection and Affordable Care Act, including its controversial "requirement to maintain minimum essential coverage." This requirement, also known as the individual mandate, forces all Americans to buy or secure health insurance under what Congress claims is its power "to regulate commerce…among the several states."

Twenty-six of those states, plus the National Federation of Independent Business and several individuals, are challenging the health care law, claiming it is an illegal power grab by the federal government that tramples the Constitution and undermines the principles of federalism.

Contrary to what Nancy Pelosi would have you believe, these challengers have a strong and serious case. Here are four of their best arguments against the individual mandate.

4. The Individual Mandate Threatens the Foundations of Contract Law

American contract law rests on the principle of mutual assent. If I hold a gun to your head and force you to sign a contract, no court of law will honor that document since I coerced you into signing it. Mutual assent must be present in order for a contract to be valid and binding.

This view was widely shared by the framers and ratifiers of the U.S. Constitution. Here's how Pennsylvania lawyer James Wilson, a signer of both the Declaration of Independence and the Constitution, put it in one of his legal lectures:

The common law is a law of liberty. The defendant may plead, that he was compelled to execute the instrument. He cannot, indeed, deny the execution of it; but he can state, in his plea, the circumstances of compulsion attending his execution; and these circumstances, if sufficient in law, and established in fact, will procure a decision in his favour, that, in such circumstances, he did not bind himself.

The individual mandate turns this longstanding legal principle on its head. After all, there's nothing mutual about the government forcing you to enter into a binding contract with a private company. As the Institute for Justice, the public interest law firm that pioneered this argument, explains in the powerful friend of the court brief it filed in the case, the framers of the Constitution "would never have given, and in fact did not give, Congress, through the guise of the Commerce Clause, the power to gut the foundation upon which the entirety of contract law rests."

3. The Individual Mandate Cannot Be Justified Under Existing Supreme Court Precedent

Defenders of the individual mandate will tell you that of course Congress has the power to compel every American to buy health insurance from a private company. "Under an unbroken line of precedents stretching back 70 years," argues liberal University of California law professor Erwin Chemerinsky, "Congress has the power to regulate activities that, taken cumulatively, have a substantial effect on interstate commerce."

It's true that the Supreme Court has greatly expanded Congress' regulatory powers. In the 1942 case of Wickard v. Filburn, the Court held that the Commerce Clause allowed Congress to forbid an Ohio farmer named Roscoe Filburn from growing twice the amount of wheat permitted by the Agricultural Adjustment Act and then consuming that extra wheat on his own farm. In 2005, the Court reinforced this decision, holding in Gonzales v. Raich that medical marijuana cultivated and consumed entirely within the state of California still counted as commerce "among the several States" and was therefore open to federal regulation.

Yet neither of those precedents stretched the Commerce Clause so far as to allow Congress to regulate inactivity—such as the non-act of not buying health insurance. As the National Federation of Independent Business argues in its brief, "uninsured status neither interferes with commerce or its regulation nor constitutes economic activity. Instead, the uninsured's defining characteristic is their non-participation in commerce."

The Supreme Court has never before granted Congress the unprecedented power to regulate inactivity under the Commerce Clause. If the Court sticks to its own precedents, it won't do so now.

(Click below to watch Reason.tv's "Wheat, Weed, and ObamaCare: How the Commerce Clause Became All Powerful.")

2. The Individual Mandate Rests on an Unbounded and Unprincipled Assertion of Federal Power

Does the Commerce Clause allow Congress to do anything it wants so long as an economic activity is remotely involved? Under the government's theory of the case, yes, congressional power is essentially unlimited. As the D.C. Circuit Court of Appeals remarked in its ruling on the individual mandate:

The Government concedes the novelty of the mandate and the lack of any doctrinal limiting principles; indeed, at oral argument, the Government could not identify any mandate to purchase a product or service in interstate commerce that would be unconstitutional, at least under the Commerce Clause.

Solicitor General Donald Verrilli will need to come up with something better than that when he argues the case before the Supreme Court. As the multi-state challengers put it in their Supreme Court brief, "there is no way to uphold the individual mandate without doing irreparable damage to our basic constitutional system of governance." At a minimum, the Court's conservatives will expect the solicitor general to lay out a plausible limiting principle for congressional power under the Commerce Clause. If Verrilli does not—or cannot—do that, the individual mandate is in big trouble.

1: The Individual Mandate Violates the Original Meaning of the Constitution

Article 1, Section 8 of the U.S. Constitution grants Congress the power "to regulate commerce…among the several states." The framers and ratifiers of the Constitution understood those words to mean that while congress may regulate commercial activity that crossed state lines, Congress was not allowed to regulate the economic activity that occurred inside each state. As Alexander Hamilton—normally a champion of broad federal power—explained in Federalist 17, the Commerce Clause did not extend congressional authority to "the supervision of agriculture and of other concerns of a similar nature, all those things, in short, which are proper to be provided for by local legislation." In other words, the Commerce Clause was not a blank check made out to the federal government.

Yet in its decisions in both Wickard v. Filburn and Gonzales v. Raich, the Supreme Court held otherwise, allowing Congress to regulate the wholly intrastate cultivation of wheat and marijuana, respectively. Those decisions cannot be squared with the original meaning of the Commerce Clause. As Justice Clarence Thomas remarked about the majority's reasoning in Raich, "If Congress can regulate this under the Commerce Clause, then it can regulate virtually anything—and the Federal Government is no longer one of limited and enumerated powers."

Unfortunately for constitutional originalists, Thomas is unlikely to persuade a majority of his colleagues to wipe the slate clean by overturning Wickard and Raich. But as I explained earlier, the Supreme Court already has sufficient reason to strike down the individual mandate without touching any of its existing precedents. That approach—which targets the mandate's unprecedented regulation of inactivity—could satisfy both Thomas and his faint-hearted originalist colleagues on the bench. If five or more justices are interested in expressing at least some fidelity to the text of the Constitution, the individual mandate is finished.

Damon W. Root is a senior editor at Reason magazine.