For about the first third of the 20th century, the U.S. Supreme Court was under the sway of a doctrine called “substantive due process.”

The court routinely invoked this (and the related concept of “freedom of contract”) to overturn laws attempting to regulate business on matters such as working conditions, minimum wages, and health and safety.

This was what conservatives now call “judicial activism,” except that the activist judges ― imposing their unwanted doctrines on an unwilling democracy ― were conservatives.

In more recent decades, conservatives, at least in their rhetoric, have turned firmly against judicial activism. According to the 2008 Republican platform, “Judicial activism is a grave threat to the rule of law because unaccountable federal judges are usurping democracy, ignoring the Constitution and its separation of powers, and imposing their personal opinions on the public. This must stop.”

The campaign against the Obama administration’s health-care reform ― in particular, the effort to get the individual mandate declared unconstitutional ― will soon test that commitment to restraint in the Supreme Court.

Federal district courts have ruled both ways on the subject, and so have two federal appeals courts. Now it is up to the nine justices.

If they rule the Affordable Care Act unconstitutional, it will be the clearest sign yet (and there have been earlier ones) that conservatives on the court and off will not be satisfied with overturning what they regard as liberal rulings from the Earl Warren and Warren Burger Courts (most notably Roe v. Wade, the abortion decision). They intend to begin a new era of conservative judicial activism, imposing their agenda through the courts, just as they have long accused liberals of doing.

It may not come to that, because the argument that the law is unconstitutional is pretty thin. It went virtually unmentioned during the debate before the law was passed, and previously failed to exercise conservatives when something like it became law in Massachusetts under Governor Mitt Romney. It has the aroma of confection about it, as if someone at one of the conservative think tanks was reading the law and suddenly said, “Eureka!”

The argument turns on the fact that the U.S. government is one of delegated and enumerated powers. It only has the powers specifically granted to it by the Constitution. Every law must have a “hook” somewhere in that document. Ever since the New Deal, that hook has generally been the clause authorizing the federal government to regulate interstate and foreign commerce.

The courts have interpreted the Commerce Clause broadly. Between 1935 and 1995, not a single law enacted by Congress was held to exceed its power under the Commerce Clause. Even the civil rights acts of the 1960s were held to be constitutional because of the effect of racial discrimination on commerce. (Which, in reality, was hardly the point.)

The 1995 ruling involved a law forbidding guns near schools. A subsequent ruling overturned the federal Violence Against Women Act. Neither of these had anything to do with commerce.

The argument against the Affordable Care Act concerns the individual mandate ― the requirement that every American either have health insurance (whether from an employer or purchased individually) or pay a fine or tax. Without something like this, the system (or any universal insurance plan other than single payer ― that is, a real government takeover of the health insurance system) won’t work. You need healthy people in the system to help pay for the unhealthy ones, and for themselves when, inevitably, they turn unhealthy.

Opponents say there is nothing in the Commerce Clause that enables the federal government to require commerce, by forcing every citizen to purchase something. But the court has previously found that the clause allows the government to regulate how much wheat a farmer can grow, even if he doesn’t sell it.

Commerce Clause jurisprudence is a mass of scholastic distinctions. Is the individual mandate a tax or a fine? Is medical care commerce or not? (We know that insurance is commerce, because a case long ago said so.) Is it constitutional for states to mandate auto insurance but not for the federal government to mandate health insurance? You can weave your way through these thickets and emerge with an argument that the Commerce Clause doesn’t support the individual mandate. But it would be a real stretch.

A Supreme Court decision to throw out health-care reform on Commerce Clause grounds could open up hundreds of other federal laws to legal challenge ― starting with the Civil Rights Act. It’s hard to believe that this court really wants to relitigate the second half of the 20th century, but we’ll have an answer soon enough.

Meanwhile, let’s hear a bit less about the evils of judicial activism from politicians who are eager to have judges throw out one of the most important pieces of social legislation since the civil rights era.