In my opinion the failure to counter the left's explanation of the financial debacle doomed McCain. Immediately after the crash, the left flooded the media with the message (lie) that conservative Republicans aka greedy bankers had solely created the calamity. McCain did nothing to counter that lie, in fact he reinforced it, and he and Palin reiterated the lie about greedy bankers.
If McCain had immediately gone on tv with a clear, forthright explanation of how the Dem-controlled FM fiasco was the culprit, he might have had a chance. Before the disaster McCain/Palin were leading in the polls. One week after the catastrophe, they were behind and never caught up. Millions of Americans bought the Dem lie about conservative responsibility. Too little, too late did McCain mention the role of the Dems, and Obama in the scandal.
My wife, who pays little attention to politics says the debates killed McCain because he came off as a weird old man.
I think if he would have voted NO on the bailout it would have been huge. He came out strong against earmarks and then helped write and voted for a bill full of them.