Months ago I predicted that McDonald v. Chicago may wind up in a 4-1-4 split. At the time, I queried:

What happens if there is a 4-1-4 split as follows:

• Chief Justice Roberts, Justice Scalia, Justice Kennedy, and Justice Alito vote to incorporate the Second Amendment through the due process clause and strike down the Chicago law
• Justice Thomas rejects incorporation through the Due Process Clause, and chooses to extend the right to keep and bear arms (cf. 2nd Amendment) to the states through the Privileges or Immunities Clause, and strikes down the Chicago Law
• Justice Stevens, Justice Ginsburg, Justice Breyer, and Justice Sotomayor, uphold the validity of substantive due process incorporation but find that the Second Amendment should not be incorporated, and finds the Chicago law constitutional

Under the “Marks Rule” Marks v. United States, 430 U.S. 188 (1977), what would the precedent be in such a situation?

So what is the most narrow rule?

Professor Cohen at Faculty Lounge provides some insights:

In order to apply Marks we have to understand what the Court means by “narrowest grounds.” As Max Stearns has so well articulated in an article advocating including Marks as a part of the first year constitutional law curriculum, the rationale behind Marks is that the opinion with the “narrowest grounds” is precedential because it is logically entailed by the opinion of enough other Justices to make a majority who agree on the point. In Bakke, Justice Powell’s opinion on diversity being a compelling reason for affirmative action is logically entailed by the opinion of the four Justices who believe that affirmative action is constitutional in a wide variety of circumstances. Thus, his opinion is controlling on this point because it is, in essence, supported by a majority of the Court (even if the other Justices did not sign on to his opinion).

This same analysis is inapplicable in cases of the voting paradox. Justice Thomas’ opinion is not logically entailed by Justice Alito’s (just like Justice Alito’s is not logically entailed by Justice Thomas’). After all, they both completely reject each other’s rationales, so there can’t be any overlap. This is true in all voting paradox cases — the two positions that make up the majority vote on the outcome do not overlap and one does not logically entail the other.
…
Thus, Marks cannot apply to McDonald because there is no “narrowest grounds” implicitly agreed upon by the five Justices in the majority. This is true in all voting paradox cases.

So the question remains, on remand to the 7th Circuit, what will Judges Posner, Easterbrook, and Bauer do with this? Knowing Posner and Easterbrook, the Court almost certainly will rely on the plurality approach of using Due Process. But if this issue is raised in different circuits, could other courts rely on privileges or immunities as the “most narrow ground.” We shall see