Conservatives won a substantial victory on Thursday. The physics of American politics — actions provoking reactions — continues to move the crucial debate, about the nature of the American regime, toward conservatism. Chief Justice John Roberts has served this cause.

The healthcare legislation’s expansion of the federal government’s purview has improved our civic health by rekindling interest in what this expansion threatens — the Framers’ design for limited government. Conservatives distraught about the survival of the individual mandate are missing the considerable consolation prize they won when the Supreme Court rejected a constitutional rationale for the mandate — Congress’ rationale — that was pregnant with rampant statism.

The case challenged the court to fashion a judicially administrable principle that limits Congress’ power to act on the mere pretense of regulating interstate commerce. At least Roberts got the court to embrace emphatic language rejecting the Commerce Clause rationale for penalizing the inactivity of not buying insurance:

“The power to regulate commerce presupposes the existence of commercial activity to be regulated. . . . The individual mandate, however, does not regulate existing commercial activity. It instead compels individuals to become active in commerce by purchasing a product, on the ground that their failure to do so affects interstate commerce. Construing the Commerce Clause to permit Congress to regulate individuals precisely because they are doing nothing would open a new and potentially vast domain to congressional authority. . . . Allowing Congress to justify federal regulation by pointing to the effect of inaction on commerce would bring countless decisions an individual could potentially make within the scope of federal regulation, and — under the government’s theory — empower Congress to make those decisions for him.”

If the mandate had been upheld under the Commerce Clause, the court would have decisively construed this clause so permissively as to give Congress an essentially unlimited police power — the power to mandate, proscribe and regulate behavior for whatever Congress deems a public benefit. Instead, the court rejected the Obama administration’s Commerce Clause doctrine.

The court remains clearly committed to this previous holding: “Under our written Constitution . . . the limitation of congressional authority is not solely a matter of legislative grace.”

The court held that the mandate is constitutional only because Congress could have identified its enforcement penalty as a tax. The court thereby guaranteed that the argument ignited by the mandate will continue as the principal fault line in our polity.

The mandate’s opponents favor a federal government as James Madison fashioned it, one limited by the constitutional enumeration of its powers. The mandate’s supporters favor government as Woodrow Wilson construed it, with limits as elastic as liberalism’s agenda, and powers acquiring derivative constitutionality by being necessary to, or efficient for, implementing government’s ambitions.

By persuading the court to reject a Commerce Clause rationale for a president’s signature act, the conservative legal insurgency against Obamacare has won a huge victory for the long haul. This victory will help revive a venerable tradition of America’s political culture, that of viewing congressional actions with a skeptical constitutional squint, searching for congruence with the Constitution’s architecture of enumerated powers.

By rejecting the Commerce Clause rationale, Thursday’s decision reaffirmed the Constitution’s foundational premise: Enumerated powers are necessarily limited because, as Chief Justice John Marshall said, “the enumeration presupposes something not enumerated.”

When Nancy Pelosi, asked where the Constitution authorized the mandate, exclaimed “Are you serious? Are you serious?” she was utterly ingenuous. People steeped in Congress’ culture of unbridled power find it incomprehensible that the Framers fashioned the Constitution as a bridle.

Now, Thursday’s episode in the continuing debate about the mandate will reverberate to conservatism’s advantage. By sharpening many Americans’ constitutional consciousness, the debate has resuscitated the salutary practice of asking what was, until the mid-1960s, the threshold question regarding legislation. It concerned what James Q. Wilson called the “legitimacy barrier”: Is it proper for the federal government to do this? Conservatives can rekindle the public’s interest in this barrier by building upon the victory Roberts gave them in positioning the court for stricter scrutiny of congressional actions under the Commerce Clause.

Any democracy, even one with a written and revered constitution, ultimately rests on public opinion, which is shiftable sand. Conservatives understand the patience requisite for the politics of democracy — the politics of persuasion. Elections matter most; only they can end Obamacare. But in Roberts’ decision, conservatives can see the court has been persuaded to think more as they do about the constitutional language that has most enabled the promiscuous expansion of government.