In the Trinity College study American Nones: The Profile of the No Religion Population (PDF), we find that the Republican Party has troubling times ahead. The "nones" are those described as "no religion" in the American Religious Identification Survey 2008. The "American Nones" study has this to say about "nones":
Nones are easily misunderstood. On the one hand, only a small minority are atheists. On the other hand, it is also not correct to describe them as “unchurched” or “unaffiliated” on the assumption that they are mainly theists and religious searchers who are temporarily between congregations. Yet another incorrect assumption is that large proportions of Nones are anti-rationalist proponents of New Age and supernatural ideas. As we will show, they are more likely to be rational skeptics.
Amongst some of the highlights of the survey, it found that in 1990, 6% of Democrats were nones and 6% of Republicans were nones. Today it's 16% Democrats and 8% Republicans. Nones are currently 15% of the population, but that number is expected to rise to about one fourth of the population in 20 years. Further, fewer than 10% of nones under the age of 30 are Republicans. If the present trends continue, the Republicans are going to find it harder and harder to sway voters.
In short: the Republicans are actively chasing away one the largest and fastest growing demographic groups in the US. I wonder, perhaps, if they're actually responsible for this group's growth.