First, Barack is supposed to be the candidate of "change." So when he starts acting like just another negative, attack-dog pol, it diminishes the warm glow of the Obamessiah's halo.
Second, "Senator 'Sweetie'" already has trouble with women, who wonder about the disparity in pay between men and women in his Senate office and his dismissive "You're likable enough, Hillary" in one of the debates. So why does it make sense for him to try to diminish Governor Palin, what with his talk about her being "a fine mother and an up-and-coming public servant" (irony alert, coming from a guy with 143 days in the US Senate) and a mayor of a small town?
Third, what is the Democratic presidential candidate doing attacking a vice presidential candidate? Looks a little weak, no? Just as the discussion comparing his record and Governor Palin's reveals his painful lack of experience, the fact that he's going after a VP nominee reveals the "stature gap" between him and McCain.
Finally, doesn't the fact that Joe Biden isn't making these attacks -- probably because the campaign rightly senses that a huge "foot in mouth" moment will be the result -- raise some question about Barack's judgment in choosing a vice president who can't even go after his GOP counterpart effectively?