The Decline of Evangelical America
I'll just note that the author of this article, John Dickerson, was my Theta Kappa Nu society chaplain at Bob Jones, and is an all around great guy. I quibble slightly with his analysis, but it captures nicely the generational mood.
It is the cycle. Every time christianity becomes obsessed with political power, it becomes spiritually weak. Even the author seems to have some trouble viewing evangelicalism as having influence in society wihout political power being part of it.
I commend to you all "Less than Conquerors: How evangelicals entered the 20th century" by Douglas Frank. He makes this case brilliantly. Out of print, but generally available used, it will change how you look at influence and power in society.