Avi Feller and Andrew Gelman unpack the relationship between voting and income. They find "only a strong red America-blue America split toward the top of the income distribution" and that toward "the bottom, the electoral map is a sea of blue":
Why does this happen? Our research on opinion poll data from earlier elections finds that lower-income Americans tend to vote based on economic issues, while richer voters consider social issues as well as economics in their voting decisions. This is sometimes called post-materialism: the idea that, as individuals or groups become more comfortable, they can afford to think beyond their immediate needs.
The so-called culture war between red and blue America is concentrated in the upper half of the income distribution, and voting patterns reflect this.
Yglesias frames these splits differently:
It’s always the case that the poor vote for Democrats and the rich vote for Republicans (but by a smaller margin) while the middle class is contested terrain. But compare the GOP’s best year (2004) to its worst (2008) and you’ll see a dramatic shift among low-income voters to the Democrats. Whether you want to attribute that to fundamentals or message or whatever, the same point holds—the shifts are pretty broad-based and whatever it is that makes a party more popular with the middle class probably also makes it more popular with the rich and the poor.