If I recall from campaigns where I was too young to vote, it was common for the running mate of a candidate to take the role of "Hatchet man." This allowed the main candidate to be positive and on message, while still giving a voice to all the negatives about the otherside.
The Republicans don't really need this as they have FOX news and various other rightwing media outlets to do it. But the Democrats desperately need this. When our nominee is becoming clear it is very important that the running mate be choose based on his or her ability to full fill this function. With all the under reported scandels there is lots of stuff that needs to be brought out.
We need someone that can apply the words treason (Plame outing) and cyberterrorist (Congressional computer files) to the Republicans!