Quite apart from being gratuitously demeaning towards Barack Obama, and seeming (even though unintentionally) to evoke racism (slaves were considered property rather than humans, after all. People bidding at slave auctions in Charleston during the years of slavery, would say "I'll take that one!"). It was also quite obvious to me that it was a "Hillary moment." Let me explain.
It has seemed so preposterous that this young black man, who has come onto the scene so recently, has stolen both their thunder! It seemed utterly impossible to both of them.
It became obvious that both Bill and Hillary were completely taken aback that people would be drawn to this "untested" senator, who was hardly even known barely four years ago. It doesn't make any sense! And that, I believe is where this "that one" moment came from, for McCain.
An older guy. An American hero. The "maverick" and "true compromiser" with political opponents, who has had a history of "reaching across the aisle," suddenly is feeling, and seeing before his eyes, that this guy, "that one," is actually winning, and he, like Hillary before him, is having a meltdown moment.
This is why McCain (and Hillary before him), have had meltdowns in their campaigning styles. And it's also quite enlightening that Barack Obama has had the drive, steadiness, and apparently totally unflappable temperament, combined with obviously extraordinary organizing skills, and an amazing ability to learn and adapt throughout this campaign, in order pull this whole thing off. And you can almost hear Hillary, Bill and now McCain saying "Who the hell is this guy? - Yes, 'that one!'