Iowa State University political scientist Dianne Bystrom has been keeping tabs on the political ads this campaign. Bystrom says the candidates haven’t used many so-called “negative” ads.She says Steve Forbes did run some negative ads, which she says he probably needed to do more of since he was behind. On the democrat side, Bill Bradley waited 10 days before getting an ad on the air responding to a charge made in an Al Gore ad. Bystrom says that’s an eternity in politics, and it let voters form an opinion on the issue without a rebuttal.Bystrom is director of the I-S-U Center on Women in Politics.