On the radio yesterday a House Republican explained that though he didn't expect his party to do well in the next election he expected that sooner or later "the pendulum would swing back the other way" allowing his party to come back to power. This metaphor is a common one but I think it's the hides more than it reveals. A pendulum implies that the various political parties/philosophies stand still while the voting public swings about. The reality is that the political parties and their ideologies are the changing too.
Of course they're changing! We're learning more and more everyday. From how to best administer health care to how regulatory capture can warp well-intentioned plans; voters are going to form different opinions based on new information and experiences. Moreover, the actual problems facing society are changing all the time. Many of them won't require government intervention but some of them will. Any political party that puts ideological fealty over pragmatically grappling with relevant issues is going to be toast electorally.
An Example: When facts on the ground show that "preemptive" war and unilateralism don't work, that creates an opportunity for either party. On the left that meant passing over those who supported the war to nominate a candidate who opposed it from the beginning. On the right it's lead to a renewed interest in isolationists (or "non-interventionists") like Ron Paul. One can only imagine this interest increasing if the right meets more electoral defeats. Either way the parties are changing every bit as much as the populace is.
At first glance the history of American politics might look like a fickle populace swing back and forth between two parties. But the party labels disguise the constantly changing ideologies underneath.
So in conclusion let me offer a new metaphor for democratic politics. Voters are like a laser-pointer spot moving randomly around the room and the parties are like two cats trying to catch it.