Why poor whites vote against their own self-interest is indeed material. It is the critical question which many of us have been trying to answer, since DuBois wrote his book. That was the central question of my book, what I called Labor, Race and Immigration - California's Unholy Trinity. It was also the central question of Alexander Saxton's exploration of anti-Chinese racism in The Indespensable Enemy UC Press 1975).
Unless we understand this - how racism develops and how it is sustained even when irrational - we will never be able to solve the problem of racism.