The legal argument from the Obama administration in support of the Affordable Care Act has always been straightforward: the Commerce Clause empowers the federal government to regulate interstate commerce; the American health care system is interstate commerce; and the Affordable Care Act regulates the health care system. Ergo, the ACA fits comfortably within the confines of the Commerce Clause. Q.E.D.

The right, narrowing in on the individual mandate, says this is all wrong. The government has the power to regulate interstate commercial activity, including health insurance, conservatives argue, but those who choose not to buy coverage aren’t engaging in an activity; they’re engaging in inactivity. Therefore they fall outside the law’s reach, and the mandate is unconstitutional.

This isn’t just some tangential point; it’s largely the heart of the right’s case against the law. To knock down the “inactivity” argument is, as a general matter, to reject the larger legal challenge.

And as we saw today at the D.C. Circuit, even Judge Laurence Silberman, a right-wing Reagan appointee, just isn’t buying it.

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. But the Government does stress that the health care market is factually unique; there are few other markets, it says, where participation is a virtual certainty, or where declining to buy a product disproportionately causes a national economic problem. […]

To be sure, a number of the Supreme Court’s Commerce Clause cases have used the word “activity” to describe behavior that was either regarded as within or without Congress’s authority. But those cases did not purport to limit Congress to reach only existing activities. They were merely identifying the relevant conduct in a descriptive way, because the facts of those cases did not raise the question — presented here — of whether “inactivity” can also be regulated… In short, we do not believe these cases endorse the view that an existing activity is some kind of touchstone or a necessary precursor to Commerce Clause regulation.

Silberman soon after explained that those challenging the health care law “cannot find real support for their proposed rule in either the text of the Constitution or Supreme Court precedent.” That’s usually a bad sign.

As for the activity/inactivity question, the ruling addresses the debate exactly as the left has been all along. Yes, there may be folks who don’t want to buy insurance, and they would be penalized under the law. But under our system, those folks still get sick, still go to the hospital with medical emergencies, and — here’s the kicker — still get care. As you may have noticed, for quite a while, it’s been one of the right’s favorite arguments: the uninsured can always just go the emergency room and receive treatment, whether they have insurance or not.

Of course, when the uninsured get this care, and can’t pay for it, the costs are passed on to the rest of us — it makes the entire system more expensive, with hospitals and medical professionals providing care without compensation from the patient. As a consequence, those who would choose not to get coverage have a significant impact on the larger health care system, which is precisely why the notion of a mandate enjoyed broad, bipartisan support up until late 2009. There was never any doubt as to its constitutionality.

Indeed, as Jonathan Cohn emphasized, to say Congress lacks this power is to say that Congress lacks the authority to approve the Civil Rights Act under the Commerce Clause. Even Silberman isn’t willing to go that far: “[The health care mandate] certainly is an encroachment on individual liberty, but it is no more so than a command that restaurants or hotels are obliged to serve all customers regardless of race.”

The question now is whether five U.S. Supreme Court justices agree.

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Follow Steve on Twitter @stevebenen. Steve Benen is a producer at MSNBC's The Rachel Maddow Show. He was the principal contributor to the Washington Monthly's Political Animal blog from August 2008 until January 2012.