I agree with Matt:
This business of Travis Childers winning an extremely Republican district on the heels of two other Democratic special election wins drives home how infuriating the idea of even having an extended "electability" argument about "who can win" is at this point. The reality is that given current conditions, either Clinton or Obama is very likely to win. That, I assume, is why Clinton is fighting so hard. There's no need to join David Corn in reaching for esoteric explanations, she's fighting hard for the prize of the nomination because it's a very good prize to have.