To work, negative ads must be believable. To accuse an opponent of being soft on child molesters won't work. It lacks credibility. One cannot ask voters to believe such ill of an opponent that he deserves not just defeat but imprisonment. But to say that he puts his perception of constitutional rights ahead of convicting child molesters does work.
Paint a picture. Negatives must be thematic. John McCain, in the current campaign, is too scattershot, one day hitting Barack Obama for his Chicago political connections and then accusing him of vapid celebrity the next. It is only when the negative campaign paints a consistent picture that it can work.
Some political consultants, including most Republicans, treat positive advertisements like the overture before the show begins, marking time until the real campaign starts and the negatives begin to hit. That's wrong. Positive ads that explain a program, develop a theme, or spell out hot-button issues are still the most effective communications in politics. But negative ads work and have their place. They are how the voters find truth in a morass of claims and counterclaims. With much of the media oriented toward the left or the right, negative ads are often the only way voters can penetrate the claims of the various campaigns and get the facts.
Voters always tell pollsters that they hate negative ads, but politicians continue to run them. That's because the same polls show that they work. In a world with flawed politicians, we need negative ads; otherwise, we won't know candidates' defects until it's too late.