Much is made of the influence of religious conservatives in the Republican Party. The mainstream media has been fairly obsessed with the topic for a generation.
Arthur Brooks wrote this article for National Review Online on the influence of secular Americans on the Left in general and the Democratic Party in particular. Here's an excerpt:
Secular liberals, and especially those who are explicitly nonbelievers, have become a major force on the political left. Researchers have found, for example, that delegates to the Democratic National Convention — the politically-active folks who nominate the Democratic candidate for the U.S. presidency — are more than twice as likely to be completely secular as the population-at-large.