Andrew Gelman shows 2008 figures that indicate that there is virtually no relationship between income and self-reported ideology in the U.S. electorate.
This finding is subject to the usual caveats: namely, that everyone thinks they’re moderate, that most people really don’t think about politics in ideological terms (at least in the broad categories of “liberal” and “conservative”), and that these ideological terms themselves are fuzzy categories capturing multiple underlying political orientations to begin with. But at first blush it does contradict the accepted wisdom.