How will the state of the economy determine the outcome of the 2008 election?

Lynn Vavreck, UCLA political science professor, says that if the economy is not growing, the incumbent party is not likely to win a presidential election, regardless of who the parties nominate.  Even in the face of wars throughout history, the economy is the single most important issue in an election.  The public is not divided on this issue.  People prefer prosperity to decline.  There are only four cases in history when the alternative party did not win when the economy was bad.