The moment he lost it -- which is to say, this campaign's version of, "I won't hold my opponent's youth and inexperience against him": Historians won't say that it was when John McCain chose Sarah Palin. They will point to the following words:
"Today, John McCain put the former mayor of a town of 9,000 with zero foreign policy experience a heartbeat away from the presidency." With those words, Barack Obama, like the Russian sub captain in The Hunt for Red October, torpedoed his own boat by mistake. Whether the damage is fatal remains to be seen; but that it was both unbelievably stupid and devastatingly damaging, is I think beyond question.
A whole post, maybe series of 'em, coming on that one.
The fatal character flaw that doomed him: Vanity. (I think I'll do a whole post on that one, too, someday when I have time.) Not pride; not arrogance. You must remember that vanity, while a vice, has this endearing and pitiable quality that pride and arrogance do not have: it is based on a core of deep insecurity. Obama is not pride or arrogant; he is vain, a proposition I hope at some point to have time to defend with lots of evidence drawn from his behavior over the years. And, tragically, he was put into a position in which vanity was, of all the character flaws he could possibly have suffered from, the one most capable of sinking all his hopes.
The crippling misfortune he was unable to overcome: His utter inability to understand any American who does not come from the extremely provincial far-left academic subculture into which he was born, in which he was raised, and in which he has spent all his life. There is no subculture in America that makes more noise about diversity; but there is no subculture in America whose members are more smugly ignorant of anyone other than their own kind, nor less interested in relating with those others on a level of genuine mutual interest and equality (Obama and his friends can patronize the common man, or they can sneer at him, but they do not understand him and, even worse, have no idea how deeply out of touch they are and how severely limited they are by their own provincial ignorance). The greatest mismatch in this campaign is not the mismatch between McCain's and Obama's experience, vast as that mismatch is, nor between McCain's and Obama's oratorical ability, vast as that mismatch is. It is the Great-Rift-sized gulf between how well McCain and his team understand Obama's team and the swing voter, and how utterly Obama's team fail to understand McCain's team and those same swing voters.
Those words, that flaw, and that misfortune may not be enough to sink even as incompetent a campaigner as Obama in this Year That Should Be Of The Democrats (at this point I'd say it's a pick-'em); but if he does indeed go down to ignomious defeat (and in this year, any Democrat who can't beat a Republican in a national election is deserving of ignomy and permanent retirement from politics), then I believe history will deliver an analysis pretty much along the lines I just laid out briefly here in this post.