The January 31 ruling by US District Judge Roger Vinson of Florida that the new health reform law is unconstitutional in its entirety was immediately hailed by Republicans and Tea Party activists, who have made overturning the law their chief goal. The second federal district court judge to invalidate President Obama’s health care law, Judge Vinson reasoned that if the Commerce Clause empowers Congress to require citizens to buy health care insurance as a means of regulating “interstate commerce,” there would be no limit to Congressional power. It could require us to buy cars, bread, or even broccoli, as all could equally be said to be economic actions that would promote commerce. Surely, Vinson maintained, there must be some limit on Congress’s power. Thus, he concluded that neither the Commerce Clause, which gives Congress the authority to regulate “interstate commerce,” nor the Necessary and Proper Clause, which authorizes any appropriate means that might further that goal, affords Congress authority to regulate “inactivity,” by requiring those who can afford it to purchase health insurance.

Yet just two days later, on February 2, Charles Fried, Harvard law professor, Solicitor General under Ronald Reagan, and one of the country’s leading conservative lawyers, told the Senate Judiciary Committee that Judge Vinson’s rationale was clearly wrong, and that the health care law is plainly constitutional.

Fried’s testimony carries particular weight, not only because of his conservative credentials, but also because he was counsel for the challengers in one of the only two cases in the last sixty years in which the Supreme Court has found a federal law beyond Congress’s authority under the Commerce Clause: United States v. Morrison. In that case, the Court ruled that Congress could not regulate violence against women because such violence was not a commercial or economic activity. Fried recognizes that the Commerce Clause has limits, but he testified that Congress was well within those limits when it enacted the health reform law, known as the Patient Protection and Affordable Care Act.

Fried began with the undisputed proposition that health insurance is interstate commerce, and therefore a proper subject of Congressional regulation. Requiring individuals to purchase health insurance, Fried argued, is in turn justified because it is an appropriate means to further the legitimate aim of reforming health insurance. Without such a mandate, the law’s most important and popular safeguard, which prohibits insurance companies from denying coverage to people with pre-existing medical conditions, would be unworkable, as people would have no incentive to purchase insurance until they are already sick. The Necessary and Proper Clause permits Congress to adopt any appropriate means to further Congress’s unquestionably legitimate end of regulating health insurance. Fried characterized the reasoning of opponents such as Judge Vinson, who maintain that the law impermissibly regulates “inactivity” rather than “activity,” as “entirely wrong and even worse quite confused.” In a rare show of bipartisanship on this most partisan of issues, Fried was joined in his defense of the law by Walter Dellinger, who was Acting Solicitor General under President Bill Clinton.

But does this mean Congress could compel us to buy broccoli? As I argue in a new essay in The New York Review, “Is Health Care Reform Unconstitutional?”, there may well be a limit to what Congress can require the citizenry to do, but that limit is not found in the Commerce Clause. That clause is designed merely to allocate law-making power among the federal and state governments, not to safeguard any particular sphere of individual liberty. And the Court has long interpreted it to authorize Congress to regulate all economic activity, no matter how minor or localized, on the theory that in our integrated national economy, all economic activity affects “interstate commerce.” There are, of course, liberty-based constraints on what Congress and the states may do, found in the Bill of Rights. But none of them would even conceivably stop Congress or the states from requiring people to purchase health insurance as part of a collective solution to a collective problem, especially as there is ample evidence that those who don’t buy health insurance effectively shift the cost of their inevitable care to taxpayers, hospitals, and the insured—in 2009 alone, to the tune of $43 billion.

Like US District Judge Henry Hudson of Virginia, who declared the health care law’s individual mandate unconstitutional in December, Judge Vinson appealed to what at first blush might seem an intuitively attractive notion—all other things being equal, it is more troubling for the government to tell us what we must do than to tell us what we must not do. As both Judges Vinson and Hudson put it, Congress may regulate “activity” that affects interstate commerce, but not “inactivity.” The intuition underlying the activity/inactivity distinction, however, rests on notions of individual liberty, not the allocation of power between the states and the federal government, the focus of the Commerce Clause. If there are limits on what the government can force us to buy, they are found in the Due Process Clause, which prohibits state and federal governments alike from infringing individuals’ liberty without “due process.” But the Republican judges, congressmen, attorneys general, and other opponents who challenge the law have invoked the Commerce Clause, not “due process.”

The opponents invoke the Commerce Clause rather than individual rights for two reasons. First, there is simply no precedent for the proposition that “due process” bars governments from mandating the purchase of insurance, where the failure to do so predictably imposes costs on others. Massachusetts already does so. The Social Security Act compels all workers, whether they like it or not, to contribute to a government retirement program. And in 1905, the Supreme Court held that states may, consistent with due process, require citizens to undergo a compulsory medical procedure—vaccination—in order to avoid the spillover effects that their contracting smallpox might inflict on others. Here, Congress’s mandate is much less invasive, and is similarly justified to avoid the spillover effects that the uninsured will have on the national health care system—and on the taxpayers and insured who will ultimately have to foot their health bills.

Second, “due process” as a substantive constraint on government power is anathema to the very conservatives who object to the health care law. It is that provision, after all, that famously protects the right of women to have an abortion, and of consenting adults to engage in sexual relations without state interference. And it is that right that is most closely associated with charges of “judicial activism,” for the text of the Constitution offers no guidance as to which intrusions on liberty violate “due process” and which do not.

In fact, the paradigmatic example of inappropriate judicial activism involved the use of “substantive due process” to invalidate economic regulation. In Lochner v. New York, the Supreme Court in 1905 struck down New York’s law mandating maximum hours for bakers, finding that it violated due process because it infringed the “liberty of contract.” The most activist Court in history struck down hundreds of laws on this theory, before it repudiated Lochner in the mid-1930’s, recognizing that the Constitution does not mandate any particular form of economic regulation, but instead leaves that subject to the political process.

Seeking to sidestep Lochner and a due process argument that would have no appeal to the conservative judges they need to attract, the health care law’s opponents argue that the individual mandate instead exceeds Congress’s power under the Commerce Clause and the Necessary and Proper Clause. But as former Solicitor Generals Fried and Dellinger argued on Wednesday, that argument is “entirely wrong.” Now that the health care law’s opponents have won two lower court victories, they will have to defend their position on appeal—and ultimately before the US Supreme Court. But in the end, the only way they can prevail is by appealing to the very “judicial activism” that they are so accustomed to denouncing.