It’s not a Dem definition, it is the term avowed white supremacists used to describe themselves and their movement going back as far as the early 20th century.
The reason they chose to use that phrase is for the exact reason you and others are unwittingly citing as a defense in this thread. “I’m white, I’m a nationalist, no shame in either of those”, and there isn’t. The white supremacists then used these two true definitions to create a new definition to help legitimize the idea that if you are white and believe in America that it was entirely acceptable to extend that to the belief that whites are the superior race. It was calculated specifically to help legitimize the movement.
There’s a reason that Hitler often cited US laws and race relations in the early years of his movement. The efforts to explain and justify the institutionalized racism in the US were a great inspiration to the Nazis and were seen as a model to follow by Goebbels and others. It’s not by chance that the first foreign book published in Germany after the Nazis came to power was written by an American.
en.wikipedia.org
Our history of whitewashing and justifying white supremacy is very well established. You don’t have to go very far back in time to find textbooks that reference the kindly and paternalistic relationship between slave owner and slave. Organizations like the Daughters of the Confederacy worked diligently to reframe race relations in a way to eliminate the shame of white supremacy and those efforts were incredibly successful. It was a full century after the Civil War until we meaningfully and legally addressed much of the institutionalized racism present in our legal system.
So you can pretend none of that happened and claim ignorance to the usage of the term “white nationalist”, by white supremacists, for over a century, but it won’t change the actual definition.