The data do not support this thesis. McCain was behind for almost all of the campaign, apart from a brief post-convention bump. Here's the Pollster graph for the period in question:
You will note that McCain's slide began September 7, a week before Krauthammer claims; and Obama's re-surge began September 9. Pollster's polls are smoothed out, but the turning point was well before Lehman, and correlates with the disastrous
All along, the clear line for McCain was always down, and only the convention period - when people were still under the temporary illusion that Sarah Palin was a credible, rather than a farcical, candidate - gave McCain any hope.
Charles also argues that Palin hurt McCain solely because she undermined his "experience" card. This is part of the truth, but much more important was not that she was inexperienced but that she was barely at a high-school education level, unable to tell fact from fiction, and totally out of her depth. That called McCain's judgment into question, and highlighted his impulsive, reckless decision-making style. When that jumpiness was confirmed by his campaign suspension (done, some are now saying, to distract from Palin's Couric mess), and by his terrible debate performances, the cake was baked.
I general agree with Andrew's assesment. I'd just like to add that the week before Lehman's fall was when we experienced the first real backlash against McCain's very negative campaigning, criticizing his ads on sex education, his criticism of Obama's talking about lipstick on a pig etc. That was when the media stopped giving a free pass to John McCain and started calling him on his "lies". I think that contributed to the beginning of his downfall. It also changed the journalist's perspective of John McCain for the rest of the campaign.
Edit: Maybe I should've read the next post on Andrew's blog, he posted a reader comment that is very similar to what I wrote.