Both parties do this, Nat. I agree with Blue that there is too much party coercion to vote a certain way by both Democrats and Re4publicans. I think that is part of what people mean when they say there are very few "statesmen" today. Instead of representing what their respective constituencies want, most pols kowtow to their party line. And, yes, groups like "the Tea Party" and abortion opponents and religious organizations apply pressure to pols to advance their agendas. So too do groups such as "Move On" and "Occupy" and ACLU apply their pressure. To only look at one side of this is to deny a big part of the total picture.
I think what looks like compromise is really still traditiionally liberal positions. It is just that some Republicans gave up the centrist-right position years ago to try to stake out a clear difference from Democrats. For example, what is "centrist" about universal health coverage that forces every citizen to participate or face fines? We used to call that "socialized medicine", and a large majority of Americans opposed that. Now, any person that wants to deny anyone govt-sponsored health coverage is deemed a bigot or other shade of hater. Same with illegal immigration: What is centrist about seeking amnesty for millions of people who entered US illegally? Media likes to describe this issue as split down the middle with American voters, but that all depends upon how the question is phrased. I don't think most Americans want to give amnesty and essentially make our borders meaningless -- that is a very liberal position. The problem is that Republicans went so far right that they allowed Dems to first occupy the political middle and then drag their liberal views to the fore and present them as moderate/centrist.
There are other damaging things the Republicans did, I admit that, but I think to look at these alleged compromises as moderate positions is to ignore recent history. I don't believe Americans have really changed so much that they abandon the ideals of individualism, personal initiative and responsibility, standing on your own two feet, making your own way, adherence to customs, traditions and civility.
Remember: Speaking of immigrants, there are still people who come to this country on small boats, are penniless, don't speak English, and certainly do not look like WASP Americans, but within 5 years they own their own businesses and their families are prospering here . . even in these difficult times. Meanwhile, a lot of native-born Americans can only see obstacles to their success. Our "need" for govt assistance/coverage is an illusion of our own making. If foreigners can prosper here with all the disadvantages that we don't have to face, then we should be able to succeed on our own without govt. propping us up. We just have to make better decisions -- take advantage of free education/libraries, develop our personal work ethic, stay away from intoxicating substances, avoid having babies too young, learn how to dress for success and how to sell yourself to others. These are the things Americans always did that brought them success. But today, too many Americans are lazy, make bad decisions and have lousy attitudes, and then expect the rest of us to bail them out (wow, I sound like my accountant friend!)