When the U.S. Supreme Court hears oral arguments later this month on whether the Patient Protection and Affordable Care Act’s individual mandate, which requires all Americans to buy or secure health insurance, oversteps Congress’ lawful authority to regulate interstate commerce, the Obama administration will be drawing heavily from the legal arguments of a surprising ally: conservative Justice Antonin Scalia.

That’s because in 2005, when the Supreme Court last heard a major Commerce Clause challenge to a federal regulation, Scalia sided with the liberal majority and wrote a sweeping opinion in favor of federal power. In that case, Gonzales v. Raich, the Court held that the cultivation and consumption of medical marijuana entirely within the confines of the state of California still qualified as “commerce...among the several states” because this intrastate use of medical pot “substantially affects” the interstate black market in the drug.

Justice Clarence Thomas found the majority’s reasoning specious, and famously stormed in dissent, “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.”

But Justice Scalia wasn’t so worried about limiting the government’s reach. Congress “may regulate even noneconomic local activity,” Scalia wrote in his concurring opinion, “if that regulation is a necessary part of a more general regulation of interstate commerce.” Scalia also distinguished his vote in Raich from two recent decisions where the Supreme Court had endorsed a more limited scope for the Commerce Clause. In the first of those cases, United States v. Lopez (1995), the Court struck down the Gun Free School Zones Act because possessing a gun within 1,000 feet of a school was not an economic activity and therefore not open to regulation by Congress via the Commerce Clause. Then, in United States v. Morrison (2000), the Court extended this line of reasoning to void the Violence Against Women Act, which had relied on the Commerce Clause to impose federal criminal penalties for gender-based violence.

Since Scalia had voted with the majority in both Lopez and Morrison, many Court-watchers expected him to be equally skeptical of Congress’ power to regulate the local use of medical marijuana. But when it came time to decide Raich, Scalia broke with Thomas and the other conservatives and instead argued that the two limiting precedents did not apply. “Neither [Lopez nor Morrison] involved the power of Congress to exert control over intrastate activities in connection with a more comprehensive scheme of regulation,” he argued.

Scalia even reaffirmed the Supreme Court’s controversial 1942 ruling in Wickard v. Filburn, which had allowed Congress, as part of its power to regulate interstate commerce, to forbid an Ohio farmer from planting twice the amount of wheat permitted under the Agricultural Adjustment Act even though that extra wheat was going to be consumed entirely on the man’s own farm. In Scalia’s view, even that degree of local activity was reachable via the Commerce Clause. “The potential disruption of Congress’s interstate regulation,” he wrote of Wickard, “and not only the effect that personal consumption of wheat had on interstate commerce, justified Congress’s regulation.”

All of these arguments by Scalia appear in the legal brief the Obama administration recently submitted to the Supreme Court in defense of the Patient Protection and Affordable Care Act. In fact, Scalia’s reasoning is cited extensively in order to bolster the government’s claim that the individual mandate is a perfectly legitimate exercise of congressional power. But will the government’s play for Scalia’s vote pay off?

There’s reason to think that it won’t. In January 2011, as the legal challenge to the individual mandate was heating up, the Supreme Court declined to take up a case known as Alderman v. United States. At issue there was whether Congress could forbid violent felons from buying, owning, or possessing body armor under its Commerce Clause power. Justice Thomas dissented from this decision, arguing that by failing to take up the case, “the Court tacitly accepts the nullification of our recent Commerce Clause jurisprudence,” including U.S. v. Lopez. What’s notable is that Scalia signed on to Thomas’ dissent, suggesting that despite his vote in Raich, Scalia would still like to see some limits imposed on congressional power.

The libertarian and conservative lawyers who crafted the legal challenge to the health care act are counting on Scalia to maintain this recent skepticism towards federal authority. As Georgetown University law professor Randy Barnett argued in the December 2009 Heritage Foundation paper that essentially launched the case against the individual mandate four months before the president signed the health care act into law, “There is every reason to believe that five Justices of the Supreme Court will be open, and perhaps even eager, to reaffirm the principles of Lopez and Morrison in a case [not involving] marijuana.”

Barnett and the other legal challengers argue that while Wickard and Raich greatly expanded Congress’ power to regulate economic activity, neither precedent stretched the Commerce Clause so far as to allow the regulation of inactivity—such as the non-act of not buying health insurance. If Scalia accepts this crucial distinction between the permissible regulation of economic activity and the impermissible regulation of inactivity, he won’t have to renounce his vote in Raich in order to find the individual mandate to be unconstitutional. If he accepts the government’s argument that Raich is broad enough to allow Congress to force every American to buy health insurance from a private company, the individual mandate will almost certainly survive.

In other words, when the ObamaCare oral arguments kick off later this the month, keep a close watch on Scalia.

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