Editor's Note: This article has been updated to reflect the Supreme Court's announcement that it would hear a case challenging the constitutionality of the Affordable Care Act.

As President Obama’s Patient Protection and Affordable Care Act has wound its way through the justice system, courts have split on the issue of whether the Act passes constitutional muster. On Monday, the Supreme Court announced that it would resolve the matter once and for all, agreeing to hear a challenge to the law that would set the stage for a ruling that will likely come in the middle of the 2012 election campaign. Already commentators are speculating about what legal opinions the Court will draw on in issuing its own verdict. But they needn't search very far: There have indeed been many lower court opinions on the issue already, but it's the most recent one—the D.C. Circuit Court’s decision last Tuesday to uphold the Affordable Care Act—that should loom larger than most for the Supreme Court.

For most court watchers, the D.C. Circuit opinion was deeply significant and a genuine surprise—and an extreme disappointment to those opposing the law—because it provided the most authoritative, truly conservative defense of the Act thus far. The conservative Supreme Court justices will find in it an opinion not only with legal merit, but also one with which they are likely to have some political affinity. The Obama administration would be wise indeed to study this opinion as they prepare their defense.

The D.C. Circuit opinion upholding the Act rests on two fundamental conservative tenets: an “originalist” interpretation of the Constitution and a restrained view of judicial power. Writing for the majority, Judge Laurence Silberman begins his substantive analysis by quoting the text of the Commerce Clause: “Congress shall have Power … To regulate commerce … among the several states.” The legal issue in the case is whether the words “regulate commerce” extend to the regulation of economic inactivity—to force people to take the action of purchasing health insurance. Employing a classically originalist approach to interpreting the Constitution, Judge Silberman does not consider what the words “regulate commerce” might mean today, but instead references Samuel Johnson’s 1773 dictionary to determine what those words meant to those who ratified the Constitution in 1789. Johnson defined “regulate” to mean “to prescribe certain measures,” or “to adjust by rule or method.” To “regulate,” Judge Silberman reasoned, “can mean to require action, and nothing in the definition appears to limit that power only to those already active in relation to an interstate market.” Judge Silberman concludes: “There is therefore no textual support for appellants’ argument.” To a true conservative, and to most everyone else, a constitutional argument that has no “textual support” in the language of the Constitution is an argument that will lose almost every time.

By grounding its defense of the individual mandate in the text of the Constitution itself, the D.C. Circuit’s opinion is far more difficult to attack than other opinions and commentaries that rely upon vulnerable Supreme Court decisions—such as the famous Wikard v. Filburn—which many conservatives believe should (and might be) overruled by the Roberts Court. For example, National Review’s Avik Roy wrote that Judge Silberman’s opinion shows how important it is “for conservatives to overturn Wickard v. Filburn.” In Wickard v. Filburn, which Roy calls “the original sin of left-wing jurisprudence,” the Supreme Court held that Congress had the Commerce Clause power to regulate the amount of wheat a farmer grew for his own family’s consumption because home-grown wheat competes with wheat in commerce and therefore affects its market price. The case is cited by the government in its defense of Obamacare as an example of Congress regulating economic inactivity that has an impact on the economy.