Sometimes voters respond negatively even to fair attacks. That's why in multicandidate races, an attack by candidate A on candidate B can hurt A as well as B, and end up helping candidate C or D.
That's why many campaigns hesitate before attacking. And it also gives them a motive to make attacks that can be sustained because they are accurate and fair.
Third, advertising is not always decisive. Other things can matter more. The barrage of negative ads against Gingrich hurt him in Iowa and New Hampshire, but in South Carolina (which has not yet voted as I write) it did not prevent him from overtaking first Santorum and drawing even with Romney in the polls. Debate performances trumped attack spots.
Behind the disdain of the high-minded for negative campaign spots is a fear that they will erode Americans' faith in politics and government. These folks like to cite polls showing Americans once had great confidence in institutions and that now they lack it.
But polls have been showing lack of faith in institutions going back to the late 1960s. The only time when pollsters found high levels of confidence was when the questions were first asked in the 1950s. That was during the two decades when American institutions -- big government, big business, big labor -- enjoyed enormous prestige after they led the nation to victory in World War II and presided over the unexpected growth and prosperity of the postwar era.
I strongly suspect that if you could go farther back in history and ask those same questions, you would find that during much of our history, most Americans were grousing about politicians and complaining about government. Mark Twain and Will Rogers made good livings doing so.
In any case, negative campaigning will persist. Those who enjoy wallowing in negative ads should fly to Florida, find a TV and keep clicking the remote control.