The Supreme Court's recent decision in Danforth v. Minnesota addresses an interesting issue in constitutional federalism: Is it constitutional for state courts to retroactively apply a newly announced federal constitutional protection for criminal defendants even if the US Supreme Court holds that such retroactivity isn't required by the federal Constitution? In a 7-2 decision, the Court decided that state courts can use state law to apply federal constitutional protections retroactively even if the federal Constitution doesn't require such a rule. Interestingly, the seven justice majority includes the four liberal justices and the three most conservative ones (Alito, Scalia, and Thomas). Chief Justice Roberts and moderate conservative Justice Kennedy dissented.

I think the Supreme Court majority got it right. Chief Justice Roberts' dissent argues that the need for "uniformity" in the application of constitutional rights forbids states to do this. However, the federal Constitution sets a floor for individual constitutional rights, not a ceiling. States are free to provide defendants with broader rights than the U.S. Constitution requires. They are also free to interpret state procedural law in a way that applies federal constitutional protections more broadly than the federal courts believe to be constitutionally required.

I rarely agree with Justice Stevens on federalism issues. But I think he gets it exactly right in this passage from his majority opinion:

There is, of course, a federal interest in "reducing the inequity of haphazard retroactivity standards and disuniformity in the application of federal law." [quoting the dissent] This interest in uniformity, however, does not outweigh the general principle that States are independent sovereigns with plenary authority to make and enforce their own laws as long as they do not infringe on federal constitutional guarantees. The fundamental interest in federalism that allows individual States to define crimes, punishments, rules of evidence, and rules of criminal and civil procedure in a variety of different ways — so long as they do not violate the Federal Constitution — is not otherwise limited by any general, undefined federal interest in uniformity. Nonuniformity is, in fact, an unavoidable reality in a federalist system of government.

UPDATE: I have corrected my silly mistake of misreading Minnesota as Missouri. I probably got confused because Senator John Danforth is a well known lawyer and former senator from Missouri and I intuitively associate the name "Danforth" with his state even though the Danforth in this case has no connection to the former senator (so far as I know).