I think there are two approaches to take to this. The first is to back to the 18th or even 17th century and change things about pre-Independence American society. The second is a 20th century mad dictator who holds white supremacists beliefs.
With the 18th century POD, the key is to find a way to prevent abolition of slavery in the northeast. Find a way to have African slaves imported to do what indentured servants do. I suspect you have to keep both the Congregationalists/ Puritans and the Quakers out The first states to abolish slavery were Pennsylvania and the New England states, it took longer in New York and New Jersey. Then as other countries abolish slavery, have the fact that the United States is one of the few countries in the world to still have slavery to become a perverse point of national pride, in fact the powerful USA of teh 20th century uses its influence to re-introduce slavery into countries that don't have it.
If you can keep slavery on a nationwide basis, you keep the slave codes and all that entails. You may have to find a way to kill or marginalize Jefferson to keep any Enlightenment ideas from gaining much currency. Paine and Franklin can just settle permanently in Europe.
The other POD is the time honored AH POD of FDR getting assassinated, a fascist regime comes to power, and a dictator comes out on top with white supremacist beliefs (though this is counter-intuitive, its not a given with fascists), and you are off to the races. However this could get reversed easier by just having the regime fall from power.
Immigrants are a problem. I don't think keeping out non-white immigrants, or importing non-white laborers and then treating them only a step above the actual slaves, is that difficult with the 18th century POD. Our 20th century dictator could introduce a South African style racial gradient system to deal with those of Asian and Latin American ancestry.
However, 18th century British immigration policy was pretty liberal at least for white people, so if you really want to keep immigrants from Catholic and Orthodox countries out that is difficult. And of course with the 20th century POD they are already here. Making them second class citizens doesn't work because they presumably won't come (or stay) under these conditions, and once they are here they are too numerous.