First, let us look at some of the factors that account for the formation of our political attitudes. These include such things as race, income, gender, educational level, and religion.
Next, we are left to ask how these things can cause cleavages in public opinion. Basically, they can do this because people differ so widely in terms of these attributes. The fact that our political opinions can be influenced by our educational level, for example, creates a cleavage between those who are and are not well-educated. In addition, we should note that these factors can lead to cross-cutting cleavages. For example, a well-educated person tends to be more likely to be liberal. However, if that well-educated person is religious, he or she is going to be pulled in different directions since religiosity tends to be correlated with conservatism.
In these ways, the factors that influence the formation of our political attitudes also lead to cleavages in public opinion.