It doesn't necessarily follow that radical fundamentalism in America would contain a purely religious overtone. Within our system people do advocate violently without the extreme physicality we see in other nations, but I am sure we are no different in our emotions and nature than are others. We have a system that allows a venting of public frustration...a system some are advocating dismantling, without thinking through the inevitable repercussions that would surely follow.
Newt Gingrich is a case in point who used to brag that 'we are the radicals' and would do most anything to win including tearing apart the fabric of institutions that though flawed, kept the US from imitating the disfunction(s) other nations.
I could go on in detail, but few here care for serious in depth discussion on that level.
Just as some would claim radical secularists are bent on destroying religion in this country (yes there are a few) some claim radical religious fundamentalists would destroy all vestiges of secularism and some would. The problem with both is they tend to wallow in hyperbole and claim it's fact.
Yes, thankfully we do have a system, as flawed as any man made system is, that allows us to vent, rail and at times actually discuss the differences and attempt to reach a compromise. Granted even that doesn't always work as it should, current politics being a case in point, but it does have the effect of minimizing the shift to extremes more often then not and we as a people tend to be more pragmatic and defensive of our liberties to allow the radicals to gain the upper hand.
I'll go out on a limb and claim the vast majority of Christians and secularists in this county reject the fundamentalists on both sides and that the primary reason for their (the fundamentalists) press is the media itself which revels in controversy, no matter how small. It sells.