The Seventh Circuit Court of Appeals recently decided McDonald v. City of Chicago, a challenge to Chicago’s gun ban. The case has major implications for protecting gun rights at the state level, but its importance goes further than that. Depending on what the Supreme Court does, it could make originalism — relying on the text of the Constitution and its amendments as they were understood when enacted — the accepted standard for interpreting the Bill of Rights, rather than the whims of a handful of justices.

The plaintiff’s case in McDonald is based on the Second Amendment, but also on the Fourteenth. Last year, in Heller v. District of Columbia, the Supreme Court ruled that the Second Amendment protects an individual’s right to own firearms against infringement by the federal government. But can a state or local government infringe that right? This question hinges on the constitutional principle of “incorporation” — the notion that the Fourteenth Amendment makes the states subject to the Bill of Rights.

When it was enacted in 1791, the Bill of Rights applied to the federal government only. Individual states could (and did) restrict free speech, for example, or have an established church. The states were beholden only to their own laws and constitutions and to certain provisions in Article I of the U.S. Constitution. After the Civil War, as Justice Clarence Thomas wrote in Zelman v. Simmons-Harris (2002), “the Fourteenth Amendment fundamentally restructured the relationship between individuals and the States and ensured that States would not deprive citizens of liberty without due process of law.” But what “liberty” was included in this guarantee, and what was meant by “due process”?

The full text of the first section of the Fourteenth Amendment reads:

No State shall make or enforce any law which shall abridge the privileges or immunities of citizens of the United States; nor shall any State deprive any person of life, liberty, or property, without due process of law; nor deny to any person within its jurisdiction the equal protection of the laws.

Ringing phrases, to be sure, but somewhat short on specifics. In fact, the meaning of every major clause in the first section is disputed, and has been since the amendment’s enactment in 1868.

Case law provides little clarity. The first major Supreme Court decision to grapple with these questions came in the Slaughterhouse cases of 1873, but — to quote Justice Thomas again, this time from his dissent in Saenz v. Roe (1999) — that decision “all but read the Privileges or Immunities Clause out of the Constitution.” By adopting a narrow interpretation of the clause, the Court gave states wide latitude to enact laws they thought were necessary.

Towards the end of the 19th century, however, as various reform movements took hold, restrictive state laws came to be seen as an obstacle to progress. So the justices eventually concluded, as Prof. James W. Ely of Vanderbilt Law School notes, “that the Fourteenth Amendment did confer a national standard of rights against the states.” But instead of reviving the “privileges or immunities” clause to enforce this standard (and thus reversing Slaughterhouse), they seized upon the “due process” clause.

That clause may simply seem to restate the Fifth Amendment requirement guaranteeing an individual’s right to a day in court and the protections of the legal process, this time applying it to the states as well as the federal government. That’s the way many modern originalists understand the clause. But as reform took hold, courts began interpreting it to mean that states had no power to deprive citizens of important rights, whether or not those rights were mentioned in the Constitution. The judges themselves would decide what rights fell under the clause’s protection. They called this doctrine “substantive due process” to distinguish it from the traditional day-in-court meaning, which came to be called “procedural due process.”

At first, substantive due process was used mostly to reverse state encroachments on economic choices, like freedom of contract. If a baker wanted to work 70 or 80 hours a week, no one could stop him. But as the doctrine evolved, justices used it to incorporate selective provisions of the Bill of Rights, making them enforceable against state governments (the Second Amendment, among other provisions, was excluded for various reasons). And as the 20th century wore on, these two trends diverged: Economic regulation became popular among the progressives on the Court, while regulation of individuals’ “private” conduct became even less so. The justices modified the doctrine to fit their new preferences.

The result, Ely notes, was to create “an artificial division between economic rights, which the pro–New Deal court wanted to reject, and personal rights, which they wanted to expand to mean virtually anything.” Since the 1940s, the concept of substantive due process has been greatly expanded by the Court, not just to protect the Bill of Rights from abridgment by state action, but to create rights to privacy and abortion, a “wall of separation” between church and state, and many other inventions of the Warren and Berger Courts.

Many originalists, such as David Forte, co-editor of The Heritage Guide to the Constitution, have rightly argued that “due process was never meant to have a substantive meaning,” but merely a judicial one. This argument provides the basis for many conservatives and originalists to stand against the whole principle of incorporation. In a strict interpretation of this view, Chicago is entitled to restrict gun rights, since the Second Amendment (and the rest of the Bill of Rights) does not apply to it.

To parry this objection, the briefs in McDonald that argue for incorporation rely on recent scholarship that justifies incorporation on originalist grounds. Prof. Michael Kent Curtis, of Wake Forest School of Law, who worked on an amicus brief in McDonald, used the broad text of the Fourteenth Amendment, statements made in the 39th Congress (which passed the amendment), and the legal theories of its sponsors to conclude in his 1990 book No State Shall Abridge: The Fourteenth Amendment and the Bill of Rights that incorporation through the “privileges or immunities” clause was part of the amendment’s intent.

Other originalists, such as Jim Bond, professor emeritus at Seattle University School of Law, have questioned how widespread the agreement on incorporation was, especially in the southern-state ratifying conventions that meticulously reviewed the amendment before passing it. But there is still good reason to believe, as Forte says, that some “federal package of rights is protected by the privileges or immunities clause.”

Exactly what rights are included in that package is unclear, but the connection of incorporation with the “privileges or immunities” clause provides another weapon for civil-liberties lawyers. That explains why both the progressive Constitutional Accountability Center (a “living Constitution” advocate) and the libertarian Institute for Justice filed amicus briefs in McDonald v. Chicago in favor of incorporating the Second Amendment. In a case of politics making strange bedfellows, the amicus briefs put those two groups on the same side as the National Rifle Association, which is co-plaintiff with McDonald.

In a decision delivered earlier this week, the Seventh Circuit upheld Chicago’s ban on firearms, but the decision left the merits of incorporation through the “privileges or immunities” clause to the Supreme Court. Alan Gura, arguing on behalf of the NRA and McDonald, anticipated this; he noted in oral arguments that his side intends to “preserve this argument for the upper [Supreme] Court.” (Gura was also the lawyer for the plaintiff in Heller v. D.C.)

Assuming the Supreme Court agrees to review McDonald, its decision could send shock waves through constitutional law. If the “privileges or immunities” clause becomes the new justification for incorporating the Bill of Rights into state law, the days of “substantive due process,” and all the judicial overreaching it has brought, could be numbered. Still, the fact that some liberals support this interpretation is worrisome. Would it merely substitute a new all-purpose tool for legislating from the bench in place of the old one?

Not necessarily. Basing decisions on the text-based “privileges or immunities” clause, rather than the judge-created doctrine of “substantive due process,” would naturally lend itself to the increased use of originalist analysis of the Fourteenth Amendment. The focus of inquiry would be, in Forte’s words, “what could have been reasonably understood to be the ‘Privileges or Immunities’ of Federal Citizenship by the amendment’s framers” — limiting the ability of future justices to “find” new “rights” protected by it. So no matter how the incorporation debate shakes out, an endorsement of originalism would be a victory for conservatives who prize intellectual honesty in constitutional interpretation.

Seemingly aware of these implications, the Left is trying to preserve the contrivances of “substantive due process” in an originalist guise. They want to define “privileges” and “immunities” as broadly as possible, to include what Doug Kendall of the Constitutional Accountability Center calls “very important progressive values,” such as abortion rights and same-sex marriage. The goal is to continue expanding “individual rights” while permitting restriction of property rights and economic freedoms. So if the Supreme Court decides in McDonald’s favor, it could end the controversy over gun rights but begin a host of new battles in other areas.

Yet Robert Levy, chairman of the Cato Institute, is not afraid of opening a can of worms. He says that libertarians see McDonald as an opportunity “to resurrect economic liberties suspended by the Court under the post–New Deal version of substantive due process.” Conservatives should see this case as a rare opportunity to base any incorporation of the Bill of Rights on originalist grounds — an opportunity they should waste no time in seizing, for it may not come again.

— Will Haun is a recent graduate of American University and is policy chairman of the Young Conservative Coalition. He is interning at the Heritage Foundation’s Center for Legal and Judicial Studies this summer before beginning law school at Catholic University this fall.