Several chapters of this story have not yet been written, and it would be foolhardy to assert with any certainty that the ultimate result will turn on any one particular thing. But one factor that certainly stands out is whether the Supreme Court will ultimately hold that there is a salient difference (at least under the Commerce Clause) between Congress's regulation of activity and its regulation of inactivity (and, of course, whether the minimum coverage provision does, in fact, regulate "inactivity").
I do not want to weigh in on the normative merits of this debate. Rather, the point of this post is just to identity the nature of this debate, and locate it within constitutional doctrine. Specifically, I think all will have to agree (if so forced) that there is nothing in the Constitution's text or the Supreme Court's precedents that limits Congress to regulating only "activity" when it sets out to regulate interstate commerce. Once we accept that Congress can regulate non-commercial, intrastate practices (whether we say this power comes from the Commerce Clause alone or in conjunction with the Necessary and Proper Clause), then we are forced to admit that this power allows Congress to regulate behavior that is neither "interstate" nor "commerce" in its effort to regulate interstate commerce. And once we have left the text entirely in this respect, there is no textual reason to stop at "activity."
True enough, Supreme Court decisions have used the word "activity" hundreds of times to describe what Congress can regulate pursuant to its commerce power. But again, we all must admit that the justices were not using this word consciously, in contradistinction to inactivity. Congress was regulating an activity, and thus the word was apt as a descriptor. That does not make it a salient or necessary component of any of the Court's holdings.
So if we are to find a constitutional rule that limits the commerce power to the regulation only of "activities"--circumstances in which the regulated person has taken some voluntary act to subject himself to Congress's regulatory authority--it must come from broader, structural principles. Indeed, this is what the challengers (and their amici) rely on principally. It is a pragmatic, doctrinal argument: we must draw a line somewhere, the activity/inactivity line is a reasonably administrable one, and without it Congress's authority would be limitless.
It is a strong argument in many respects. Certainly, if the Supreme Court's modern Commerce Clause precedents stand for anything, it is that there must be some limit to Congress's legislative powers--that the Constitution does not grant the national government a general police power. And to be sure, the federal government has struggled a bit to articulate the "limiting principle" in its arguments that would uphold the minimum coverage provision.
But we should not get confused about the nature and origins of this proposed rule. It cannot be objectively correct in any meaningful sense, and it does not plainly come from the text of the Constitution. Rather, it is one possible way of reconciling the Constitution's basic structural principles with the necessary breadth of Congress's authority to regulate interstate commerce in modern times. Drawing the line at "activity" may be wise as a matter of policy, and thus sensible as a judicially-created constitutional doctrine. But it is being created on the fly, in response to this particular statute, at this particular moment in political time. (To my knowledge, no one had ever articulated this distinction prior to 2009.)
To be clear, asserting that this activity-inactivity distinction is being "created" is not to denigrate it. Really. Pretty much all constitutional doctrine is "made up" in this sense--a reaction to pressing events that aims to reconcile various conflicting constitutional values, according to certain broader constitutional (and political) visions. But this just underscores that there are no right answers here, nothing that the Constitution actually commands. There are different constitutional understandings, different weights that each of us might assign to different constitutional principles (liberty versus equality, for instance), and thus different possible doctrinal solutions. For some of us, virtually limitless federal authority is a significant threat to liberty. Some of us are, by contrast, less concerned with what Congress might do unchecked by judicial review, and more concerned with what happens to a society of extreme inequality, especially as it pertains to access to health care. Most of us probably have at least some concerns on both fronts.
The Supreme Court may or may not find the activity-inactivity distinction appealing; there are good arguments on both sides. But regardless, we should understand these arguments for what they are: creative constitutional proposals, but nothing clearly dictated by the Constitution itself.