As Pew puts it:
Religious “nones” are now more numerous among Democrats and Democratic-leaning adults than are Catholics, evangelical Protestants, mainline Protestants or members of the historically black Protestant tradition.
Now, contrast that with how the religious affiliation of Republicans and Republican-leaning breaks out:
While "nones" have increased among Republicans as well as Democrats, the growth rate has been far slower -- and they remain a far smaller chunk of the overall GOP landscape than on the Democratic side.
Much of this change can be explained by two trends: (1) Millennials are far more likely than any other age group to identify as "nones" in terms of religious affiliation and (2) Those same millennials tend to identify as Democrats. (Among 18-29 year olds, President Obama beat Mitt Romney by 23 points in 2012.) More young people identifying with your party + young people as the most likely to not affiliate with any religious organization = a growth of "nones" for Democrats.
What's more fascinating to me is that the Pew study shows, again, the ways in which partisans are increasingly living in two separate Americas. Democrats are secular (or more spiritual than religious), single, racially diverse and live in cities clumped along the east and west coast. Republicans are religious, married, white and live in suburbs and rural areas largely clustered in the middle of the country.
The twain, increasingly, never meet. They watch different TV shows, get their news from different sources, shop in different grocery stores and vacation in different spots. That lack of basic familiarity with people with whom you disagree -- long a function of American culture -- makes it far easier to retreat behind a partisan flag while demonizing those who don't agree with you.
Which is where we are today. And it's hard -- given the demographic trends and hardening of partisan divide over the past decade -- to imagine any real change no matter who wins the White House in 2016.