At this point, it seems likely that Obamacare’s fate will hinge on whether Justice Roberts or Justice Kennedy come to believe there is a “limiting principle” under the government’s theory of the individual mandate. If either one, or both, can see that there is a clear limit to federal power in a universe where the mandate is constitutional, the law may be upheld. If not, the mandate could go down.

So here’s a limiting principle: Under the mandate, Congress still does not have the power to regulate activity or inactivity that doesn’t have a substantial impact on interstate commerce.

This comes by way of Charles Fried, the former Solicitor General to Ronald Reagan, a supporter of the law. In an inteview with me, he responded to the various lines of questioning advanced by the conservative bloc of justices yesterday.

“There is a limiting principle,” Fried said. “Congress can’t regulate something that isn’t interstate commerce.”

At yesterday’s hearing, Kennedy suggested that it is beyond Congress’s authority to force people to purchase something they do not want. “Can you create commerce in order to regulate it?” Kennedy asked.

Fried rejected the logic on display here. He said that under the mandate, Congress isn’t “creating commerce,” it's merely regulating how inevitable commerce will be paid for.

“You’re not compelling commerce here,” Fried said. “That assumes the commerce is the insurance. But the commerce is the health care. You’re regulating how it’s paid for. They’re not creating commerce; they’re creating the way you pay for it.”

In another potential threat to the law, Justice Alito argued that under the government’s theory — that everyone will need health care at some point — the government could also force you to buy burial insurance, since everyone dies. Another version of this argument: Since everyone has to eat, the mandate would empower government to force you to eat broccoli.

But Fried rejected this logic of unlimited government power, too. He pointed out that the theory of the mandate also requires the absence of such a mandate to have a major impact on interstate commerce. That’s the case when many people don’t have health insurance in a world where insurance companies can’t discriminate against those with preexisting conditions. It’s not the case with burials or broccoli. In neither case does the absence of mandating those purchases have far reaching effects on interstate commerce.

Alito’s argument, Fried said, “assumes that somehow not having burial insurance has a substantial economic impact. The uninsured in health care have a substantial effect for interstate commerce.”

Fried boiled down his limiting principle this way. Even in a world with an individual mandate, “Congress cannot mandate a mode of provision for payment when there’s no claim and no showing that without the mandate, there will be a substantial effect for interstate commerce.”

So there’s a limiting principle for you.