Question: "I agree that we need strict immigrations laws. However, I am trying to understand the argument for white nationalism in a county that wasn't necessarily claimed exclusively for white men. E.g., is there a moral argument against law abiding Indians/Mexicans becoming a majority?"
Answer: Yes, absolutely. It violates the most general and inarguable moral law ever to exist: the Golden Rule.
The Golden Rule says that If you don't want people who look and sound like us to export our culture to your country and shove it down your throats using your political system and your money, then we don't have to put up with that from you either, and if you push it, we have a God-given right to force the issue.
Those who have come here and failed to completely assimilate need to go home. We have the right of self-determinacy, we never voted for "multiculturalism", and we want no part of it.
This includes migrants from Africa, South Asia, East Asia, Latin America, and the Middle East. It even includes Israel - we let Jews into the US long before 1948, and they have no reason to expect a belated exception now that their true feelings about us ("We hate you, goyim! Die, Amalek!") have become crystal clear.
When our guests tell us that they hate us, we have every right to protect ourselves by any means necessary. If they're entitled, then so are we.