Steven Hayward's Chart of the Day for Power Line concerns the percentages of Rs and Ds who self-report being "religiously unaffiliated." Check it out.
If you presume the population is roughly 50-50, then you can add the two end values (37.5 and10.5) and get 48, divide by 2 to get an estimate of the percentage of the total population self-identifying as "religiously unaffiliated." It's 24%. In round numbers, 1 person out of 4 claims no religious affiliation.
I believe the upward slope of Democrats who are religiously unaffiliated parallels the number of college graduates who've joined that party. People who go to college and as a consequence become more religious are the exception.
Generally, graduates leave college less religious than they arrive. Intentionally or otherwise, higher ed is a strong secularizing influence in society.
