The Willie Horton ad was not a personal attack.
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From Wiki:
Attack ads were used again by the campaign of George H.W. Bush against Democratic candidate Michael Dukakis in the 1988 presidential election. The two most famous were the “Willie Horton” ad and an ad which ridiculed Dukakis with visuals of him looking foolish while riding in a tank. The Willie Horton Ad was especially notable for how controversial it was. The ad begins with a simple statement of Vice President Bush’s support of the death penalty. Then it describes the case of Willie Horton who was a black man convicted of murder. The ad continues to explain that Dukakis’s prison furlough program (unsupervised weekend passes from prison) released Horton 10 times and on one of those furloughs, he kidnapped a young couple, stabbed the boy and “repeatedly” raped the girl. Then the ad ends with this line “Weekend prison passes. Dukakis on crime.”[2]
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The ad not only targeted the positions taken by Dukakis, it (deservedly) went after his actions:
MALE NARRATOR: Dukakis not only opposes the death penalty, he allowed first degree murderers to have weekend passes from prison. One was Willie Horton who murdered a boy in a robbery, stabbing him 19 times.