That's a good hypothesis that should be answerable if there is county-level race data.
Also, how good is "county-level" at correlating to population density? Some huge cities are their own counties, but are there many counties that have an urban seat and also outlying areas, with a significant split into two sections of similar population, but very different population density?
I would be willing to bet that there's not a single county in the US with a population density higher than the population density of its largest city. (obviously city-county mergers like NYC, Indianapolis, Jacksonville, etc would have a ratio of 1.) Indeed, most counties that have any town of > 20k people and that are not dominated by a huge city probably fit your criteria.
I'm having trouble finding data on this where all the populations are from the same time, but Forsyth and Gwinnett counties in Georgia might be counterexamples. (Their largest cities, Cumming and Peachtree Corners respectively, are hardly cities so much as just bits carved out of North Atlanta suburban sprawl.)
I don't see much supporting evidence in the article for your claim. Aside from some conjecture about race included at the end of his analysis, the article doesn't show error based on racial breakdown of the analyzed regions. I'm not sure based on the article how you're arriving at this conclusion.
One similar hypothesis that comes to mind is that perhaps ethnic/cultural homogeneity predicts voting outcomes. That might synthesize the effects of population density (since denser areas might tend to be more diverse) as well as skin color (since homogenous areas might tend to be homogeneously white).
Not true. Tarrant county, specifically mentioned in the article, would be an example of higher density whites voting more Republican than might be expected.
Tarrant county (Fort Worth TX) is basically ground-zero for what I call the "McMansion White". I define them as:
A white, suburban dwelling American, typically married with kids, making about $33,000 per year per parent and spending every dime of it on housing, transport, food, clothes and entertainment. This person is not particularly securely employed and not particularly financially secure, thus harbors a great deal of resentment for people that don't have a similar lifestyle and consumption pattern.
