The data suggest that there are fewer higher-income liberals than there are lower-income liberals but also that there are fewer higher-income conservatives than lower income conservatives. There is little correlation between income and ideology. However, the data also suggest that poorer voters tend to be more Democratic than wealthier voters, a trend that has remained true 14 years after the publishing of this article.
There must be a lot of low-income moderate Democrats and high-income moderate Republicans out there.
This is true; many low-income moderates support the Democratic Party for class reasons, and the same goes with high-income moderates and the Republican Party.