Mathematically it is simple for a bell curve centered on deists to split into a bimodal distribution of atheists and Christian fundamentalists.

It's possible that evangelicism is growing even if religiosity on the whole is declining. It's very hard to tell from these sorts of surveys, because what constitutes being religious is extremely difficult to define. Subtle differences in phrasing lead to enormous differences in outcome, e.g. 'are you a Christian' vs 'do you go to church at least once a week' could literally have a 50 point spread in the US. (And god forbid you test if people can recite the ten commandments from memory before counting them as Christian.)

And it gets even more difficult than this because most religions don't look or feel anything like Christianity, but because that's the most common religion in the US most surveys are biased in favor of religions that look like Christianity.

So you have issues like, is the Church of Subgenius a religion? Is science a religion? Is gnosticism the same as Gnosticism? etc.

There are entire college classes you can take on the history of the methodology of the study of religion, that focus on these questions like how do you know if something is a religion and how do you determine if someone is religious or not.

edit: A fun quote I snagged from Overdosed America suggesting that the excessive belief in western medicine may in fact have some commonalities with religion:

"Could it be that, although we define our era by the tremendous scientific and technological progress that is being made (particularly in medicine), our desire to believe in this narrative of biomedical progress predisposes us to uncritical belief in its real merits? In other words, might the shared belief in the potential of medical science be, in large part, our cultural mythology?

We tend to look upon myths with romantic condescension as the stories of primitive societies that provide shared meaning and hope and ease the prospect of suffering and death -- stories that are made of 'facts' that we (scientifically sophisticated as we are) know are not really true. Our belief that we are too scientifically grounded to succumb to such nonrational beliefs may, in fact, be our myth. How else can we explain the widespread agreement that statins or the new antidepressants or the COX-2 inhibitors are genuine breakthroughs that will preserve and restore our health in ways never before possible? These are our myths, merging science and hope into our shared belief."

Less political influence due in large part to shrinking numbers. Greater chance of terrorism due to increased fanaticism among those who remain. There wasn't anything like Oklahoma City in the 1950s, was there?

Christian Fundamentalism is easily traced as a response to Christian Liberalism; you can find pretty good information on both on wikipedia.

Here's a caricature of how it went down:

(mid 1800s, Liberalism): "Hey, it turns out when you look at ancient copies of the Bible from 2 different areas, there are occasional differences in spelling and word order. Therefore, the Bible is totally fiction, but teaches moderately useful lessons, so long as we understand the figurative language it's using."

(late 1800s, Fundamentalism): "What? You guys are quacks. The King James Bible is totally reliable because God specially preserved it. Therefore, there's no figurative language whatsoever, everything is literal."

It's my perception that Fundamentalism's influence is waning; I run into far fewer "fundies" than I did during my childhood (but I live in the same house and attend the same church.) In surveys I've seen, a lower percentage of people self-identify as Fundamentalist, and a lower percentage agree with certain key Fundamentalist doctrines. They may be louder than before, but I don't think they're any more prevalent.