Abortion law upheld by minority vote [UPDATED]

There have been a lot of abortion-related cases in the news lately, but last week I noticed this case from the Supreme Court of North Dakota that struck me as particularly noteworthy. The court upheld a state abortion law against a federal constitutional challenge by a 2-3 vote. That is, two justices in favor, three against, and the two won. (Original link, and more links, all via Howard Bashman.)

Apparently North Dakota has a constitutional rule that requires a supermajority to invalidate a statute, and it applies that rule not only to state constitutional claims to federal constitutional claims. My first reaction was huh, that's weird. My second reaction was can they do that?

I've read a few articles about supermajority rules generally, but none of them really seemed to get at whether a state could impose such a rule on federal claims on its own initiative. (A lot of the citations are in this recent essay by Jeremy Waldron; Waldron doesn't really answer the doctrinal question of whether such a rule can control federal claims). I also noticed a series of dissenting opinions by various justices questioning the validity of the supermajority rule, but they didn't tend to be doctrinally detailed.

So what's the best way to think about this? I'm not sure. A few possibilities:

Due Process: The Supreme Court has repeatedly held that the due process clause entitles litigants to a certain kind of unbiased adjudicator. A rule that says that challengers need to convince four of the five justices on the court is sort of equivalent to a rule that says that there are seven justices on the court but two of them have decided that they always vote against your claim, no matter what the facts and no matter what the law. Maybe that would be a due process problem, and if so maybe this is a due process problem by analogy. But only by analogy.

Preemption: State courts are generally supposed to apply federal doctrines of constitutional law when deciding federal constitutional claims. If the Supreme Court says that abortion rights are regulated by an "undue burden" test, lower courts aren't supposed to demand a "superstrong undue burden" instead. For reasons detailed by Adrian Vermeule and Jacob Gersen, doctrinal deference and voting rules can be translated into one another. So demanding a supermajority vote to overturn an abortion statute is kind of like imposing a superstrong undue burden test. But only kind of.

Testa v. Katt, Haywood v. Drown, etc.: In a series of cases that bedevil federal courts students every year, the Supreme Court has restricted the ability of state courts to close themselves off to federal claims. But in general those restrictions have targeted states that either treat federal claims less favorably than analogous state claims or states that refuse to even entertain certain claims. Neither of those is quite true here—the state applies the same deference to state constitutional and federal constitutional claims, and it's not refusing to hear the cases entirely.

If there's a better way to think about this, or a good case or article on point, I welcome suggestions in the comments.

UPDATE: Several of the thoughts and suggestions so far will merit a follow-up post, but for now I thought I'd mention just that further research has revealed the following two footnotes that briefly address this problem:

To the extent that this clause might direct the state's highest court to affirmatively enforce a state statute despite the fact that a court majority deems the statute contrary to the U.S. Constitution, this clause plainly violates the federal supremacy clause, which specifically addresses state judges and obliges them to prioritize the U.S. Constitution over a mere state statute. No other state follows the North Dakota or Nebraska model. Instead, majority rule generally prevails on state courts. For an excellent discussion, see Evan H. Caminker, Thayerian Deference to Congress and Supreme Court Supermajority Rule: Lessons from the Past, 78 Ind. L.J. 73 (2003).

Akhil Reed Amar, Lex Majoris Partis: How the Senate Can End the Filibuster on Any Day by Simple Majority Rule, 63 Duke L.J. 1483, 1488 n.8 (2014)

One could mount several types of challenges to congressional adoption by statute of a supermajority rule for the Supreme Court. First, one could argue that Article III implicitly mandates that the Supreme Court decide cases by bare-majority rule, notwithstanding silence by the text and Framers on this issue. Second, one could argue that Congress lacks delegated authority to prescribe voting protocols for the Supreme Court, though the Necessary and Proper Clause seems on its face to suggest otherwise. Third, one could argue that Article III implicitly grants the Supreme Court exclusive, or at least final, authority to determine for itself how to decide cases, including the choice of voting procedures and protocols. Fourth, one could argue that even if Congress could impose some voting protocols on the Supreme Court, Congress could not impose any protocols that would sometimes dictate the outcome or rule of law imposed in particular cases. Fifth, one could argue that this particular protocol violates separation of powers principles because it purports to aggrandize power to Congress itself. Sixth, one could argue that a supermajority protocol targeting constitutional challenges to federal statutes would require that the Court essentially decide legal issues rather than (or in addition to) deciding cases, and this would contravene the case or controversy requirement or otherwise interfere with the proper function of an Article III court. Perhaps in recognition of these at least plausible constitutional concerns, twelve of the fourteen federal supermajority proposals introduced since 1968 were couched as constitutional amendments rather than statutory dictates. See infra app. at 121-22. I consider these constitutional challenges to a congressionally dictated supermajority rule in Evan H. Caminker, Playing with Voting Protocols on the Supreme Court (unpublished manuscript, draft on file with author).

Evan H. Caminker, Thayerian Deference to Congress and Supreme Court Supermajority Rule: Lessons from the Past, 78 Ind. L.J. 73, 77 n.12 (2003)