I've just been thinking that the political spectrum is a lot like a giant rubberband, with politicians stretching it in every direction, pulling against each other. One significant characteristic of this metaphor is that only the movements of the politicians on the outside are constrained, in that it's hard for them to push the rubberband out further from the center of mass. Politicians on the inside can move around freely until they bump up against the rubberband, and those on the outside can move away from the band, possibly allowing the band itself to move inwards. For instance, as we've seen with the Bush Administration, it's easy to Republicans to become big spenders because the Democrats' constituents want the money, so the Dems can't oppose spending even if they don't like it. Likewise, as with Bill Clinton, Democrats can declare war on anyone they want because Republicans are eager to exercise American military might to protect the country.
The flaw with the rubberband model is that even inlying politicians affect the size and shape of the rubberband, so maybe we should picture the entire thing sitting atop a balanced table of some sort that shifts as the politicians move around. The problem is that I don't like the idea of a fixed fulcrum, since the political "center" moves around in correpondence with the edges of the rubberband.