But in fact the Nazis got credibility from a whole surrounding cluster of ethnic nationalist ideas and movements promoted by people who were no more Nazis than Buchanan is. In the speech you posted, the Walter Gross is appealing to that whole complex of widely diffused ideas to make Nazi race policy palatable to ordinary folk.
Pointing out that Buchanan has bought deeply into the whole ethnic nationalist mindset is not irresponsible. The key is the notion that American identity is ethnic not civic - that some kinds of people can't become real Americans because their blood is wrong. America is a civic nation, not an ethnic nation. American identity is a matter of loyalty to a civic life of ordered liberty, not of who your daddy was or where you came from.
If there is a problem about immigration, it's not that the "wrong sort" are coming here, but that we've lost the will to initiate immigrants into the tradition of American liberty in a vigorous way as part of the process of acquiring citizenship. People have always come here for a fast buck - but there was a time when we took a lot of care in making clear that citizenship meant commitment to freedom under law on the basis of the Declaration and Constitution.
You're also right that Calhoun was the fountainhead of ethnic nationalism (at the state level) in America. I've finished with the Neo-Confed wars, so I won't get started on that, but it's no accident that his biggest fans in contemporary America are left-wing group-rights advocates like Lani Guinier.
Merited repeating.
And if we have changed so as to make it harder for immigrants to assimilate successfully is that not also a sign that we should limit immigration? If it doesn't work any more, is the experiment worth continuing? It looks like it would be easier to limit immigration first, than to try to reshape our whole society to make assimilation work better. Certainly, it seems like having cheap immigrant labor to clean up after us is one factor -- though clearly not the only one -- in the decline of our character.
Very good. The "States Rights" that Calhoun and his neo-confederate heirs extol are indeed group-rights, meant to be accessible to one group (white males) and inaccessible to others (most notably, non-whites).