Lots of people are pegging Lehman Bros collapse as the point where the McCain campaign started losing, but I heard someone point out that his numbers started dropping 4 days earlier after Palin's Katie Couric interview.
But the Lehman Brothers collapse and ensuing stock market panic cemented his loss. The "fundamentals of the economy are strong" comment showed McCain as hopelessly, desperately out of touch. The sort-of "suspension" of the campaign and return to Washington to do -- whatever it was he did -- about the bailout made him look like someone who didn't have a clue what to do in a crisis. (And skipping out on Letterman while appearing on Katie Couric's show alienated a very influential person as well as raised questions about his integrity -- a huge mistake for someone who's pegged his candidacy on "Straight Talk.") And his on-again, off-again threat at the last minute to skip the first debate underscored his ineffective response to the situation.
And then afterward, when it was crystal clear that the economy was Issue #1, and probably Issues #2, #3, and #4 as well, McCain compounded his mistakes. He never put forward and created a sustained discussion of any plan to deal with the economic mess. Instead, he trotted out Joe the Plumber as some kind of proof that he supported the average American. And he called Obama a "Socialist." But what was he actually going to do? He never made it clear.