He is not a white nationalist at all. He is a nativist, a xenophobe, and a racist, which lead to white nationalists getting energized by him, but there is no evidence he has any sort of ideological desire for racial purity or separation.
This is actually the right answer on this. That doesn't make it any better though. The fact that him being all of these things enables actual white nationalists should be damning enough.
Basically the reasons that I didn't vote for Trump despite voting almost always voting Republican. He's not a white nationalist but I was put off by the fact that he was responsible for the alt-right going from irrelevant corners of the internet to displaying their hatred in the open.
Interestingly, a lot of alt-rightists like Trump as a symbolic figure, but don't think he's actually going to pursue their political agenda.
