Where does that stem from is the question?
I could be wrong, but I think pushing for a theocracy from the right wing fundies will force americans to rethink their religious belief because a theocracy goes against the american constitution. Given people an ultimative, we are one people under one god, might not be such a bright idea.
Do they honestly believe they can turn american into a theocracy, or do they have some other goal?
Well... (it's a good question!) I can't really speak for an entire mixed group of people... but my ideas on the matter...
For some people, it's not even a group-political activity, it's simply what they feel as individuals. Here is what the Bible says is right and wrong, and the law should reflect true morality, so therefore the law should be the same. (It's very simplistic thinking but seems very sensible if you are that sort of believer -- the "Law" should be the "Truth!")
For others, they might see American as losing ground morally, they don't like the promiscuity and the horrible things that people do (drugs, sex, gluttony, greed, etc.), and they feel that to reestablish laws based on God's principles will provide the sort of society that is needed to nip such sin in the bud. This is more an SJ "Garbage in, Garbage Out" philosophy -- people are malleable and need to be put into a positive environment in order to ensure they respond positively.
And they already see other philosophies as "enemies" and ultimately sinful, who thus need to be defeated. Because God wouldn't want sin, and sin is bad for people in the long run... so let's get rid of it! And if it's a war... well, Christians should be willing to fight and sacrifice for Jesus, just like he sacrificed for us. (Etcetera.) There's definitely a war mentality in some of the conservative trenches, and they're defending the homeland which has been taken over by people who don't serve God, and they need to be patriotic and fight.
So you get at least those three mentalities in place.
And I suppose you could even throw in the "godless liberal media" cliche as well as working under the scenes, which basically prejudges any knowledge that arrives through the media that might challenge one's viewpoint as simply the work of the devil / sinful mentalities (which it WOULD be... if you are of that bent). This is why much of the scientific discussion has been shut down... because anything running against the Christian group-thought can simply be attributed to the "godless people" and ignored. (Put another way, scientists believe in evolution because their hearts are hard and they don't want God to be in charge of their lives, NOT because there happens to be any real evidential basis for evolution; it's a sad truth, they think, but that's what they've been told it is...)

It's getting late... my head is swimming... did I make any sense?