Why?

It was kind of a 'My Pet Goat' moment, only in reverse.

Politico Offers 6 reasons, but the first is most telling:

1. He’s desperate.

Let’s stop pretending this race is as close as national polling suggests. The truth is McCain is essentially tied or trailing in every swing state that matters — and too close for comfort in several states like Indiana and Montana the GOP usually wins pretty easily in presidential races. On top of that, voters seem very inclined to elect Democrats in general this election — and very sick of the Bush years.

McCain could easily lose in an electoral landslide. That is the private view of Democrats and Republicans alike.

The risks of a backlash from choosing someone so unknown and so untested are obvious. In one swift stroke, McCain demolished what had been one of his main arguments against Obama.

“I think we’re going to have to examine our tag line, ‘dangerously inexperienced,’” a top McCain official said wryly.