The New York Times Magazine today has an excellent article by David Kirkpatrick on the disillusionment of evangelical Christians with partisan Republican politics. Many of the old religious right warriors are becoming disenchanted with how little they have gotten for preaching the gospel of tax cuts and the Iraq war.
There are lots of people who do not really understand politics. If you think 9/11 changed everything, you weren't paying attention; worse, you didn't think you were supposed to.
Many evangelicals are serious about the gospel at the individual, moral, person-to-person level, but just don't see how it is relevant to the social, ethical, group-to-group level. For them, politics is an intrusion on normal life, and the objective of their political action is to fix the intrusion so they can get back to their normal, apolitical lives. The religious right became a mass movement when the school prayer decision and the abortion decision intruded on their lives. They wanted a moral majority to put things back the way they were.
The political operatives who invented the Moral Majority, Inc. saw an opportunity. Their agenda was lower taxes, especially on corporations and large estates, less regulation of business, and more military and diplomatic force to promote American business and state interests. School prayer and abortion were not really their issues, but they saw a way to turn temporarily politicized religious people into a useful voting block for their primary interests.
The key point here, I think, is that evangelicals were always going to be a temporary voting block, and they were never going to permanently mobilized by the issues that mattered most to business conservatives. I have seen many a conservative Christian tie themselves in knots trying to explain why the gospel required that were cut business taxes or the "death tax" or build a larger Navy.
When apolitical evangelicals figured out that politics is mostly about material interests and always requires compromise, they would lose interest. Unlike the secular left, the religious right always has the option of giving up on politics because it is too political. Politics is an inherently bad way to preach or enact the gospel. The evangelical crackup is mostly what happens when people who are Christians by conviction and political by necessity figure that out.