Why would a correlation that exists both inside of and outside of swing states be affected by the electoral system? Something deeper is going on.
My theory is that the value of government and getting along with very different people is forced on those who live in cities. And the feeling that government doesn't do anything for you is reinforced if you live in the country. Therefore people who live in urban areas are more likely to be Democrat, and people who live in the country are more likely to be Republican. (People do move and keep their political beliefs, but that trend is pretty much universal.)
(Ironically government spends more per capita on people who live in the country than on people who live in the city. But the spending is less visible to them.)
Of course that's just my theory. But the trend exists.
My theory is that the value of government and getting along with very different people is forced on those who live in cities. And the feeling that government doesn't do anything for you is reinforced if you live in the country. Therefore people who live in urban areas are more likely to be Democrat, and people who live in the country are more likely to be Republican. (People do move and keep their political beliefs, but that trend is pretty much universal.)
(Ironically government spends more per capita on people who live in the country than on people who live in the city. But the spending is less visible to them.)
Of course that's just my theory. But the trend exists.