This reflects my impression as an outside observer, but I'm still amazed how many downright contradicting political ideologies can see themselves to be right at the heart of the Republican party. There are Republicans who claim their political views are characterized in equal weight by conservatism, libertarianism and (evangelical) Christianity, where I can't even see two of them as being ideologically compatible. I think the difference here is that they're often pretending that these ideologies are one and the same, and don't have actual plurality but something more similar to an "acceptable views" opinion bowl.You could make a case for that. But consider the extent to which the Republican party has purged its moderates. When all the socially liberal Republicans were forced out, the socially moderates started getting the boot as well. Old school Republicans of the Country Club variety, GHW Bush, Gerry Ford, they have a damned hard time being Republicans now. Before you had the hard core social authoritarians, and the balance of power in the party was the big money interests, which didn't care a lot about those social issues. But that's changed. Even Romney now must pander more to the social wingnuts. There is no place for what he was 10 years ago. Now to be a viable Republican candidate someone has to be both a social authoritarian and economically to the right of Reagan, who himself was to the right of nearly all Republicans from the 1930s through the 1970s. If you are not that far right, don't bother running for national office as a Republican. So I would say they were a somewhat large tent, that is getting smaller by the day.
On the Democratic side, there is a larger tent. You have the Blue Dogs, who are effectively where most Republicans were 20-30 years ago. And you have the liberals, who are not nearly as liberal as they used to be except on just a couple of social issues. So there's really not much place for real liberals in the Democratic party currently, but no place for them to go either.
Now for the democrats, interestingly I can't say a lot about them. They appear to be mainly characterized by trying to keep up with the topics the right sets, and trying to salvage what they can. Seriously, to actually experience the broadness that I'm sure the democratic party still has, someone there would actually have to take initiative and set the public agenda.