...[I]n recent years...partisans have come to identify with their parties in much the manner that sports fans identify with their teams. The strong views they feel on many issues do not drive their party affiliation; it is their party affiliation that drives their strong views.
There is a... piece of evidence that party identification rather than ideology is behind the growing polarization of the electorate: On a variety of unrelated issues -- gun control, the economy, war, same-sex marriage, abortion, the environment, the financial bailout -- the views of Republicans and Democrats have become increasingly monolithic. There is no reason someone who is against abortion should necessarily also be against gun control or for economic deregulation, but that is exactly what tends to happen among committed Republicans. Loyal Democrats have similarly monolithic views on unrelated issues.
"Party identification is part of your social identity, in the same way you relate to your religion or ethnic group or baseball team," said Gary C. Jacobson, a political scientist at the University of California at San Diego. This explains why, on a range of issues, partisans invariably feel their side can do nothing wrong and the other side can do nothing right. By contrast, moderates don't feel there is a yawning divide on issues because they don't identify with one party or another. Moderates, in other words, are like people who are uninterested in sports and roll their eyes when fans of opposing teams hurl abuse at each other.