No voters cast their votes based on perfect information, but richer voters are on average best informed. We develop a model where the voting mistakes resulting from low political knowledge reduce the weight of poor voters, and cause parties to choose political platforms that are better aligned with the preferences of rich voters. In US election survey data, income is more important in affecting voting behaviour for more informed voters than for less informed voters. Further, when there is a strong correlation between income and political information, Congress representatives vote more conservatively, which is also in line with our theory.