There is general agreement among political scientists and other observers of American politics that partisan polarization has substantially increased among political leaders and activists over the past several decades. The elite theory of polarization has found considerable favor among pundits and political commentators. Moreover, the politically engaged segment of the public has been growing in size in response to the increasing polarization of the parties. A number of changes in American politics and society have contributed to the rise of mass-based polarization since the 1950s. Perhaps the most important long-term trend contributing to the rise of polarization in the twenty-first century is the growing racial and ethnic diversity of the American population. In twenty-first century America we have a two-party system that consists of a racially diverse Democratic Party and an overwhelmingly white Republican Party. The diverging positions of Democratic and Republican elected officials and candidates reflect the diverging positions of those who put them in office.