Divisions

In 1994, a solid third of Republicans were more liberal than the median Democrat. Today, virtually no Republicans are more liberal than the median Democrat.

There are three forms of political polarization, and what matters most in American politics is how they interact with each other. The first is party polarization: how far Republicans and Democrats are from each other. The second is ideological polarization: how far liberals and conservatives are from each other. And the third is engagement polarization: how different the politically engaged are from the politically disengaged.

For much of American history these forms of polarization have been different. The differences between the Democratic and Republican parties were not the same as the differences between liberals and conservatives. And the differences between liberals and conservatives were not the same as the differences between the people who participated in politics and the people who didn't. 

Vox: The single most important fact about American politics, June 13, 2014