First Things has an article up that argues that the Democratic is informed by a "Secular" paradigm, just as the Republicans are now the party of traditional Christians. Two points:

1) There's "Secular," and then there is secular. I've never seen a survey that indicates more than 5% of Americans are atheists or agnostics, usually only 1-2% will identify with those labels. So the "Secular" term is actually a larger catchall, and as the authors of the First Things article state, seculars have "overarching religious worldview of their own." Rather than defining them in terms of a negation of a "religious" worldview (that is, traditional Christianity), perhaps it would be better to reconceptualize the "secular" outlook as a liberal neo-pagan[1] spiritual sensibility (most Americans who say they have "no religion" do believe in God or some "Higher Power").

2) The Beliefnet blog points out that they neglect African-American Christians, who are the large evangelical block in the Democratic Party.

[1] I say "pagan" because in a Christian monotheistic culture, many of the spiritual values espoused by non-trads, pluralism, syncretism, immanent theism and nature worship, superficially resemble ancient pre-Christian European paganism.