Religion and the Threat Effect

"Building on the "racial threat" hypothesis -- which states that as the number of African-Americans in a community increases, the more likely white voters are to support conservative candidates and oppose policies that benefit African-Americans -- Campbell set out to see whether he could identify a similar effect among evangelical voters. It turns out that, even when you control for factors like party identification, the more secular people there were within a county, the more likely that people from evangelical denominations living there would vote Republican.
In other words, the more that evangelicals saw non-religious people around them, the greater the likelihood they'd walk a straight line from the church door to the voting booth and pull the GOP lever.

[...]

Whatever the answer is, the possibility does seem real for secularism to achieve a new awakening of its own as a political and social movement. [...] Greater visibility makes it easier for the tribe to reproduce itself: The more we wear our tribal identity on our sleeves, the easier our fellow members are to spot, and the more likely we are to define membership as one of our primary criteria in mate selection and thus pass on our identity to others."

Paul Waldman, Prospect.org, June 13, 2007

See more here Religious 'Threat' In Presidential Elections" (PDF)