It is an interesting question why the right wing churches do better than progressive denominations, especially since the main appeal to the outside world is not a life commitment to hard right religion and politics, but something calculatedly quasi-secular. Also because until the late 1970s or early 1980s the evangelicals were not particularly political at all and their politics were about half-and-half liberal and conservative, just like the rest of the population. Abortion apparently has something to do with it, but why did that particular button cause such a big seismic shift?
[WS:] Max Weber had the concept of "elective affinity," which I think explains this. The idea here is that while many concepts and ideologies are "out there waiting to be noticed" - only those that dove tail with- and legitimate the overall interests of the elite that actually get noticed. And once they are adapted by the elite, they trickle down and become internalized by the masses over time. So at the end you may have a situation that the elite does not need a particular ideology to legitimate its interests anymore, the population still treats them as *the* legitimate ideology and treats is as "stock knowledge" (which in sociological lingo means a set of shared beliefs taken for granted as self-evident).
This is how Weber explained the ascent of protestant ethics during the growth of capitalism. I think that this explanations hold quite well to explain the prominent role of evangelical religiosity in the US. Hofstadter shows in _Anti-intellectualism in American life_ how evangelical religiosity was the main legitimating force of American populism that set itself apart form both European high culture and New England 'intellectual elitism.' It was basically the legitimating ideology adopted by the 'un-churched and un-cultured' masses as well as by noveau riche philistine businessmen who got rich mainly through good luck and thus disdained knowledge and high culture.
Of course, today's business is not philistine and pious anymore, but the type of religiosity that they once adopted that mixes anti-intellectualism, with public displays of piousness and populism still lingers like a bad aftertaste and forms the basis for 'elective affinity' for right wing religiosity today. Overcoming that affinity is really an uphill battle - especially today, when monopoly capital ravages anything social in this country.
Wojtek