Political parties play a central role in the democratic process. They seek to win elections by representing voter concerns, which makes those concerns central to political decision making. When social change occurs parties serve as the organization most sensitive to representing emerging public concerns so they can secure the votes of affected constituents. The last several decades provide an important test to whether parties are playing out this role. Remarkable social change has occurred. The parties have steadily diverged in the policies they advocate (McCarty et al. 2006). Do these diverging positions reflect the divisions of society? Are parties playing the responsive role we presume in representing differing concerns? If so, how has this occurred?