Stacy is right: It wasn't the economic crisis that automatically hurt McCain, but his bumbling, pathetic, mercurial, scapegoat-seeking response to it. Here is where the campaign's fundamental incompetence came in: For months and months and months it was obvious that a credit-crunch crisis MIGHT WELL occur. Even if few people thought that it would definitely hit, much less hit as hard as it did, any sentient observer knew that it was a far from unreasonable possibility. Therefore, any sentient campaign would have had a contingency plan in place in case it did happen: The campaign and candidate would have had already approved lines -- even focus-grouped language -- in the can, ready to use in case the awful eventuality occurred. And not just language, but a basic thematic approach.
This does not mean the campaign should have had specific proposals ready at the drop of a hat; after all, nobody knew exactly what form a crisis would take. But a competent candidate would have at least had a pre-formulated, thoughtful, understandable and explainable template available with which he could describe his preferred reaction to a crisis. In other words, he needed a cogent set of principles that explained his general approach to economic instability. Instead, McCain advocated replacing Chris Cox with Andrew Cuomo (!!!!!), the very person whose policies as HUD chief helped caused this mess. And, as Stacy listed, he made a bunch of other nonsensical statements as well.
Actually, a crisis like this could have played directly into McCain's hands. His campaign has been built almost entirely on his ability to be trusted in a crisis and strong as a leader. The crisis offered him the perfect opportunity to demonstrate those characteristics. Instead, as I wrote elsewhere, the American people looked for John Wayne but McCain gave us Joe Pesci.