Much of the prosperity that we citizens of these United States enjoy has occurred because we have the world's largest and freest free trade and immigration zone. Colorado cannot charge duties on goods made in California. Nor can it keep Californians from immigrating to Colorado. This is as it should be, not merely for these United States, but for the whole world. If you want to see what happens when national tribalism becomes the overweening force in people's lives, see the histories of former Yugoslavia's tribes, the tribes of most of the African continent, and the religious tribes that live on the Irish island.
Hatred brings revenge.
Revenge brings death. But that's not the kind of population growth control any compassionate human would want. If we unconstitutionally exclude people from, say, the tribalism-wracked African continent, we have shamefully abandoned our compassion.