I really wish politicians would avoid negative campaigning, but I understand why they do it. Neither candidate can afford to look weak or defenseless during an election year. If the other candidate launches attack ads, then the offended candidate needs to prove those accusations are wrong. Some negative ads really do bring out some legitimate shortcomings that should make voters question that candidate's suitability for office.
I remember those "Swiftboat" presidential race ads that made John Kerry looked really bad. As a Democrat, I felt those negative advertisements were a low blow. I wasn't sure if I believed those men who claimed Kerry wasn't an effective leader in Vietnam. But the political damage was done, and the term "swift boating" entered popular culture.