Ideology -- self-identification as liberal, conservative, or moderate -- is a major way in which the public can be politically segmented. Because Republicans tend to be associated with conservative causes, and Democrats with liberal causes, one might assume that about a third of the population is conservative, a third liberal, and a third in the middle.
That's not the case, however. The American public is significantly more likely to identify as conservative or moderate than as liberal, leaving a situation in which about 4 in 10 Americans call themselves conservative, 4 in 10 call themselves moderates, and only about 2 in 10 call themselves liberal.