I'm not sure but I think this is the first time it's actually shown a negative value. It's touched zero a couple of times but not less that I can recall.
Rasmussen doesn't track this number per se; it was invented here at FR.
The theory of course is that the number of people who have awakened to the truth and now utterly detest -21bama exceed the number of can tolerate having the TV on while he's speaking/lying on it.
Lurking dems from Washington, take heed. You better be sure that what you're trying to do is going to work, because people are waking up. We're going to have your ass for this.