Chuck Todd on the Big Factor in the 2008 election. Money quote:
There are a couple of parallel elections that we could be in the midst of right now and depending on the country's specific mood will determine the outcome. Clearly the mood of the country is down, but is it anger or depression? Is it exhaustion or fear? If it's like '68, where the country was angry but a bit fearful of drastic change, Thompson and Clinton could succeed; both are very similar to Humphrey and Nixon in that they are selling cautious change. If the mood is exhausted and depressed, like '80, Romney and Obama could prosper. Then again, if the mood is angry with an electorate demanding of change no matter the unintended consequences, like 92, then perhaps Edwards, Giuliani or even a Bloomberg will fit the bill. In fact, take note that in the three elections cited above ('68, '80 and '92), all of them saw significant third party candidacies influence the outcome of the election (Wallace, Anderson and Perot).