First, I think Progressive is a more accurate term than "liberal" (and not just because talk radio has made the word "liberal" sound like a curse word.) The two terms--conserve vs. progress parallel each other in an interesting way.
And that points to a very interesting thought about why and how people think in conservative or progressive ways. There has been research done recently that actually suggests that these two approaches are brain-based and maybe even genetic> I don't think it's necessary to go that far, since people, including myself, do change their positions.
But I was wondering about what accounts for the linkage between evangelicalism and conservatism. Part of it is related to questions of sexual morality, but the link between laws about sexual morality and conservatism as a political and economic doctrine aren't all that sensible in the first place. So let's leave abortion and gay rights out of it, and look at more classic forms of conservatism, and see why it appeals to the same people who tend to be biblical fundamentalists.
My theory is that both religious fundamentalism and political conservatism are based on the same impulse or under-lying belief: that somewhere, at some time, the truth and standards that should be followed for all time have been written down, and that our role is to take these written rules as the basis for how we behave.
Thus the Constitution and the Bible have the same kind of authority. It is authority that can be read and understood by anyone, that we all have access to and don't need priests or judges to interpret for us, and the ideal state would be one in which we all adhere to what was written in the past in as straightforward a way as possible. (Mormonism takes this to its logical conclusion and actually teaches that the US Constitution is divinely inspired, but most conservatives implicitly believe much the same thing.)So the desire for "strict constructionists" is the same as the desire for biblical literalism.
Thus the same people who believe that the ideal church would be one that was just like in the days of Paul tend to believe that if we just went back to the way things used to be in 1800, our country would be pure and free.
On the other hand, progressives are more like Catholics in some way, in that they believe that as conditions change, our response to them ought to change as well. Not only is it not possible to go back to some idyllic pristine past such as the early church or the early republic, it would not be desirable either.
The church grew from a small local persecuted minority to a world-wide dominant belief, and the US grew from 13 small isolated states to a world power, and as realities change, so must our laws and our way of living, and this is as it should be. Just as the early days of Apple computers, in someone's garage are not the Platonic ideal of Apple, neither is the incipient stages of the United States or the church an ideal state.
The desire for a clear written document that can directly guide our lives, as opposed to a living group of human beings who re-interpret and apply principles to changing conditions is based, I think, in a kind of awe of writing. Writing is so permanent, so seemingly immutable and not subject to change. But in fact, I think it is more the case that written documents are actually less stable than they seem, since they cannot simply be read and applied, but must always be interpreted, and who we are and how we live will necessarily change what we see in a written document.
So this Read-and-obey mentality seems naive to me. It doesn't really understand how reading works, and it doesn't understand how much we impose our own preconceptions on anything we read. It also gives tremendous weight to the ideas of people who are no longer with us, and overlooks the fact that the original writers, whether they were Paul or Jefferson or Shakespeare were fallible men no different than our current leaders (really!). But once their words are enshrined in writing, they take on an authority that would never have been given to the spoken words of the very same individuals.
My conclusion is that the real difference underlying two approaches to government and religion is a different concept of authority. Is authority best held in adherence to a written document that should be simply read and followed in as literal a way as possible? Or is authority inevitably vested in people who re-interpret and re-apply principles to new and changing conditions, perhaps even questioning and changing some of the original ideas as they go?
Addendum: One further bit of evidence of this being an underlying approach to things is that conservatives tend to approach even things like grammar from this same point of view. For example, the understanding of the role of a dictionary probably splits along the same lines, with conservatives seeing the dictionary as a permanent and infallible guide to how we should speak rather than something that changes as our language changes. And they would probably hold out for old shibboleths such as proper use of whom as a standard that, even if they don't meet it, "ought" to be met for philosophical reasons. They also have a visceral attraction to "phonics" as a rule-based and unfailing means of teaching reading.