You often hear liberals complain about the fact that there are so many people in red states who should be voting Democrat but don't. They're right, but the truth is actually a little more complicated. The reason is that just as many poor people in red states as blue states vote Democrat as vote Republican. In fact, even for middle class voters, there isn't that much of a difference. So what makes the red states red and the blue states blue? The answer, as in all things, is money. The rich in the red states vote Republican and the rich in the blue states vote blue. For a breakdown of how the last election would have looked and some graphs that explain this phenomena, see: http://economistsview.typepad.com/economistsview/2007/10/rich-state-poor.html.
This raises the interesting point of whether the issue of appealing to middle-class white voters is overblown. Looks like Obama may benefite more if he figures out how to appeal to upper-middle-class voters.
We can even paint the picture in greater detail courtesy of a recent NY Times article: http://www.nytimes.com/2008/09/07/magazine/07Inequality-t.html. It turns out that the greater the income inequality in a city, the more likely it is be Democratic. David Frum explains it better than I can: "Measured by money income, Washington qualifies as one of the most unequal cities in the United States. Yet these two very different halves of a single city do share at least one thing. They vote the same way: Democratic. And in this, we are not alone. As a general rule, the more unequal a place is, the more Democratic; the more equal, the more Republican. The gap between rich and poor in Washington is nearly twice as great as in strongly Republican Charlotte, N.C.; and more than twice as great as in Republican-leaning Phoenix, Fort Worth, Indianapolis and Anaheim."
I am tempted to confuse cause with correllation. In other words, do people who live in income diverse areas vote Republican, or do people who hold Republican values move to areas with less income inequality?
The answer is a little of both, but I want to argue that it may be that exposure to income inequality teaches people to vote more progressively. I'll make a historical argument: look no further than The Lion, the Witch, and the Wardrobe. Historians have argued that the welfare state arose in Britain when poor children from London were sent outside the city, during the Blitz, to live with wealthy country families. The rich were completely unaware of, and had never been exposed to, the poverty of the city's poor. Once aware, the powerful now backed the rise of the welfare state which began after the end of the war.