Do we “want to live in a world where Texas is at war with California”? Justice Elena Kagan asked that question during oral argument in a recent case. Increasingly the states seem to be answering yes.

In recent years, states have banned official travel to states whose laws they dislike. They have sent tens of thousands of migrants to cities across the country. They have made it a crime to seek certain medical care across state lines. Some have even sent National Guard troops into other states over those states’ objections. States have always competed for jobs and resources. What’s new is that many of these actions are meant to punish other states for their political choices. Professors Katherine Mims Crocker and Charles W. Tyler call this “interstate intolerance.”

In this talk, they explain where it comes from: state parties that have moved further apart, voters who increasingly despise the other side, and state elections fought over national issues. They argue that the trend won’t fade on its own and that it is dangerous. It strains the union, and it lets one state impose costs on people who can’t vote in that state’s elections.

The Constitution offers surprisingly few settled rules for fights between states. Crocker and Tyler propose three principles to fill that gap. First, Congress should take the lead in managing relations among states. Second, borders should limit how far a state’s power reaches. Third, states should treat one another as partners in one nation, not as foreign rivals.