I believe that Jonathan misconceives what is at issue and confuses the question whether there is a national interest in marriage with the very different question whether Congress has existing power to preempt state marriage laws. Let me offer some clarifying points:
1. Those of us who support a federal marriage amendment are obviously not maintaining that Congress has “plenary police power” over marriage. If that were our view, we would be pursuing an ordinary federal statute, not a constitutional amendment.
Opponents of a federal marriage amendment maintain that there is some sort of meta-policy argument against an amendment. In support of this argument, they frequently make the factually false assertion that marriage has always been a state matter. That simply isn’t so, as Congress’s enabling acts conditioning admission of a state on its permanently barring polygamy show.
2. Jonathan’s example of public education is useful. There is plainly a national interest in education—to form citizens capable of self-governance, to develop productive workers, to have soldiers who know how to operate sophisticated weapons (to cite just a few examples). How best to achieve this national interest—whether through state or federal government control, whether through public schools or school choice—is a separate question.
Imagine that some states permanently implement some disastrously bad ideas for education. Perhaps a ban on English-language instruction in the public schools. Or maybe a new system in which elementary-school education runs from ages 60 to 65 and the educators are all four-year-olds. Would the fact that public education has traditionally been a state matter be a meaningful objection to a federal constitutional amendment to override such idiocy? The idea strikes me as laughable.
Stated somewhat differently, the proposition that something important has traditionally been a state matter generally means little more than that states have been thought to have acted, within broad bounds, responsibly on the subject. If they start acting irresponsibly, it is perfectly appropriate to reconsider how the national interest in that subject can best be achieved.
3. Jonathan’s argument that “marriage has been a state matter except in those instances in which there is a clear federal nexus” doesn’t help him. Just as the “clear federal nexus” for enabling acts barring polygamy is the provision of Article IV, section 3, that authorizes Congress to admit new States to the Union, so the “clear federal nexus” for Congress’s action on the federal marriage amendment is Article V, which authorizes Congress to propose amendments to the Constitution.
4. The national interest in marriage—traditionally, the vehicle for generating and raising the next generation of Americans—is overwhelming. Obviously, there is disagreement whether same-sex marriage threatens that interest. That issue should be addressed and debated on its own merits. The claim that marriage has always been a state issue would have little bearing on that debate, even if it were true. But since it’s false, no one should make that claim or try to hide behind it.
By the way, here’s a fine essay by Adam White that explains why federalists should support a federal marriage amendment. On the broader threats posed by same-sex marriage, see the articles by Stanley Kurtz and Maggie Gallagher that I’ve previously linked to here and here.