Of course, we can all identify negative ads that undermined the political debate. The spots that ran against Georgia Sen. Max Cleland by Saxby Chambliss in 2002 might qualify because they questioned a decorated war hero's patriotism. Perhaps the "Daisy" spot qualifies -- the ad in which President Lyndon Johnson used the innocence of a young girl to suggest in an utterly inflammatory manner that Barry Goldwater would bring nuclear war.
But one or two examples can be misleading. We need to take a systematic look at the content of ads. That is what I have done, examining (nearly) all the ads aired in presidential campaigns over the last 40 years. This comprehensive look tells a very different story from conventional wisdom. For one thing, negative ads are much more likely to discuss issues than positive ads. The general view is that negative ads are just personal attacks. That is wrong. And even when the attacks do get personal, more than three-quarters of them deal with the issues of experience or honesty -- pieces of information that are important and relevant when selecting a president.
Negative ads are also much more likely to be buttressed by evidence than positive ads. Candidates can talk about supporting a strong national defense. Negative ads have to go further. It was not credible for Kerry simply to claim that the president's policies were weakening national defense; he needed to demonstrate just how Bush's policies undermined our security. Candidates face a burden of proof when they go on the attack.