The quotation is not condescending to "the lowest white man", but rather is explaining a political/propaganda strategy that was active at the time. Up until that time the racists were inter-mixed between the two parties:
And the fact that people respond to it by attacking the very groups they've been set against rather than the government and elites manipulating them proves how well it works.
See also the apopleptic rage the rural right went into when Hillary Clinton made her "deplorables" remark. Even though she was clearly and explicitly talking about the white supremacists and racists infiltrating the Trump camp - something she was entirely correct about mind you - they chose to trauma bond with the white supremacists and rally around a billionaire pedophile rather than concede a point to a "liberal" in good faith.
Liberals are going to find it very comforting that they're not one of "the lowest white men". The people the quote is referring to won't be persuaded at all. It solves nothing. There's no solution offered to the propaganda problem. It is the flip side to the culture wars that it is describing.
* https://en.wikipedia.org/wiki/Dixiecrat
but it was the GOP that actively sought to recruit them:
* https://en.wikipedia.org/wiki/Southern_strategy
The quotation was describing the creation of the very culture wars we're still living with today. And the evidence shows that this is still going on:
* https://en.wikipedia.org/wiki/Dying_of_Whiteness