First, how should one apply the term “extremist” in the context of the American religious debate? Second, what are the contours of “mainstream” religious thought in America?Those are, I suppose, interesting questions, although I'm concerned that they distract from the issues he seems to be addressing when he evaluates Michele Bachmann and Rick Perry in the terms of those questions.
The issue of religion in politics is whether religious beliefs should be imposed on nonbelievers by political means. Both Bachmann and Perry, and likely most of their adherents, seem to believe that this is essential. Whether this counts as extremism is an argument I'll leave to others. Certainly it is what many on the right accuse Islam of in the anti-sharia laws that are being proposed across the United States. If it's wrong for Islam, it's wrong for their version of Christianity, too.
So in one sense, that's all I have to say about this great debate. But I'll add a bit about "mainstream" religious thought in America. It's hard to find what the mainstream is these days. If we take numbers of adherents, Catholics plus mainstream (old-style?) Protestants probably still outnumber evangelicals and charismatics. The numbers are hard to figure out, because many people will tell pollsters that they belong to a particular church, but they seldom attend. Catholic and mainstream Protestants have developed a body of theology that has been checked and rechecked for consistency with the Bible, their foundational document. There are disagreements among them, but there are also large swathes of agreement.
The "new-style" Christians disagree with a great deal of this theology. They need to make arguments that go beyond "God told me so" in order to convince the rest of us (well, me, anyway) that their cruelty and selfishness is justified. There is a tradition of prophecy in Christianity, and they may be right that God has spoken to them. But they have an obligation to explain themselves better, especially in their zeal to impose their beliefs on the rest of us.
They certainly are louder than the old-style Christians, who I would like to hear more from them too. But is loud mainstream? Would a majority holding a clear set of views be mainstream? And even if they were a majority, one of the things the Constitution tries to protect against is the tyranny of the majority. But they seem to think that God has told them how to deal with that, too.