It's zero-sum thinking, "the pie isn't big enough and can't get bigger and I'm afraid, so I'm going to hurt other people so that I don't get hurt".

Yes. And they still remember where they are coming from and they fear that they might again lose their piece of the pie to the groups that are considered more "American", so they feel the need to prove their place in society by cheering the leader who is preaching that the pie is getting smaller and that someone has to leave the table. This fear is handed down over generations and for some families or communities it transforms into hatred. This mechanism is very often played by amoral populists because it works so well.

Many of the most disgusting and radical Democracy hating people in Trumps inner circle are Catholics by the way. Go figure.