What does the Constitution have to say about interstate relations? Well, it depends on how you ask.
One of the main topics in interstate relations is the question of what is called choice of law, which sounds very technical but fundamentally is the question of who governs--that is, which state gets to govern any given transaction.
The same kind of question comes up at the federal level--federal law versus state law--but it is dealt with by the Supremacy Clause of the Constitution, which makes clear that if a federal law is constitutional, it is controlling. (1) But there is no Supremacy Clause for state law, which has forced people who worry about this question to look harder and elsewhere for some sort of hint about which state is supposed to govern which transaction.
Now, the Supreme Court has largely abdicated any control of the topic of choice of law. And just to give a concrete example: in 1981, the Supreme Court decided a case called Allstate Insurance v. Hague. (2) A friend and learned scholar has described this case to me as one of the most indefensible Supreme Court opinions on any topic ever. It is a case that comes from the Minnesota Supreme Court, where a man named Ralph Hague was riding a motorcycle in Wisconsin and crashed. (3)
Now, once upon a time, it was well-settled that accidents in Wisconsin were always going to be governed by Wisconsin law. Classically, states followed the rule of lex loci delicti, (4) meaning the law of the place of the wrong. Many states have changed that rule over the course of the twentieth century, depending on whether the people were from other states, and so on. (5) But even under most of these modern approaches, this should still have been an easy case. Mr. Hague was in Wisconsin. He was from Wisconsin. The other driver was from Wisconsin. Pretty much everything about the trip and the accident involved Wisconsin. (6)
But the lawsuit was brought in Minnesota, which had a law that was much more favorable to Mr. Hague, and the Minnesota Supreme Court seized on basically one fact, which is that Mr. Hague worked in Minnesota. (7) Now, he was not on his way to work. The accident had nothing to do with work. But he had been to Minnesota pretty regularly, and that, plus the Minnesota Supreme Court's conclusion that their own law was just better than Wisconsin's, caused them to think that they should apply their own law. (8) This isn't something that Minnesota made up. It reflected the influence of Professor Robert A. Leflar, who wrote an influential article advocating this approach. (9)
The case went to the U.S. Supreme Court to decide whether anything in the Constitution stops the State of Minnesota and the Minnesota Supreme Court from deciding that their law is just better than everyone else's and applies even to things that have almost nothing to do with their state. But the Supreme...