I guess a lot of the problem has to do with what you mean by "proposition nation." The neo-con answer seems to be that the country itself is the proposition about liberty and equality. Therefore it doesn't matter who lives here or what our history is. I'd have to say that there is such a thing as keeping faith and being loyal to one's own people. The country can take in some immigrants, but we shouldn't think of our nation as an empty metaphysical mold to be filled with the world's population.
There are cultural links between our institutions and values and the histories we bring with us. Too much change too quickly, too many newcomers at any one time could strain or break these links.
Also, as the world comes to accept our proposition, our own reason for being is called into question. And we face a severe identity crisis that may end in the fragmentation of our country. It's better not to try to be a global, cosmic, or universal country. There is already such a global, cosmic, or universal society being built in the world and our nation may prove redundant or superceded if it links itself too closely with that society.
But surely we do have certain ideas about liberty and human worth engrained in our political culture and with us from the founding. If we did not have such ideas we would never have parted with England, nor would we have developed as we did or fought the wars we did. Understanding our country means walking a fine line. I wouldn't want to think that America had become some universal empire of liberty. But neither would I want us to forget our heritage of liberty and dignity.
American propositionism is just a way of dressing up contemporary liberal or neoconservative preferences in the respectable garb of national antiquity in order to claim that these preferences are conservative of something. Propositionists can reply that they believe in what America is based on now, but this just makes their position a matter of contemporary political preferences, which are objects of dispute, not historical grounds upon which disputes can be settled.
This bit from the article caught my eye. I don't have any trouble with the first sentence. But the second seems a little naive. Countries change over time. What we keep and what we leave behind, what we view as essence and what as accident will differ from person to person and change over time. That's not to say that "it's all relative," just that what is most significant is always going to a contentious issue. It looks to me as though we are all trying to figure out what the thread that binds us together as a nation, both in the present moment and through time, is. And it also looks as though selection and imagination (in the best sense of the word) are bound to be a part of what ever answer we give. And our author no more escapes selection and creative recollection than those he criticizes. We are bound to choose those parts of our past that suit our current ideals and make them our American heritage. Of course this makes real trouble when our ideals don't coincide. But the author selects just as those he criticizes do.
To my way of thinking one can't avoid having some metaphysical assumptions or propositions. The "concrete" portrait our author gives of early America is guaranteed to turn off very many of us, as much in a "concrete" picture of French or German history would turn off most Frenchmen or Germans today. I think we're all building selective pictures (again I don't use the word to demean the process). We're all choosing what's best in the past to connect to.
The question is how much can you do with out, and how much do we need. A neo-con like Ben Wattenberg who distills everything down to a sentence or two that could probably be accepted by a billion people or more may have an admirable creed, but he does little to define what America is about. Nor does he give us much reason to attach ourselves to our country rather than the developing global market.
My guess is that early 19th Century Americans saw themselves as a nation with propositions about liberty and equality before law. They didn't think that the proposition was the nation or could replace the nation or provide a basis for its dissolution (actually some did come to think just that, but that only proves that abstract propositions can make trouble).
In the 20th century we stopped trying to define our characteristics as a people and let those assumed characteristics define who we are. Unfortunately, this may become an ideological lockstep. I would like to think that we can still disagree about ideological matters without being proscribed as "anti-American." It also can lead to a much shallower understanding of who we are and what we stand for. The two line creed doesn't give us as much in common as an earlier and richer cultural heritage did.
But the question for now is "Where do we go from here?" I suspect that though we weren't a purely "propositional country" to begin with, governments since at least 1965 have been working hard to make us just that, and leave us with no other basis of national unity or identity.
Anyway, a good read and worth thinking about.