Note that close-ish elections are probably how the world should be. A lot of people are conservative Republicans. Barack Obama is not a conservative Republican. There's really no reason for those people to vote for Obama, no more how badly the economy may be doing. I think the only scenario in which Obama could really win in a landslide would be one in which Bob Barr starts eating away at McCain's base vote.
This article available online at:
http://www.theatlantic.com/politics/archive/2008/08/non-mysteries/49599/