An attack ad in election terms is an advertisement whose message is meant as an attack against another candidate or political party. The ad criticizes the opponent's platform, usually by pointing out its faults and contrasting them against its own platform. The ads can be very useful in shaping public opinion. One of the earliest and most famous attack ads was one used by Lyndon Johnson against Barry Goldwater. The ad opened with a young girl innocently strolling through a field and gathering flowers. It then sharply cut to an extreme close up to her eye and then an image of a nuclear explosion. The ad was shocking and disturbing, but also very effective. It convinced many that Goldwater's more aggressive approach to fighting the Cold War could result in a nuclear apocalypse.

Attack ads were used with great success by the campaign of George H.W. Bush against Democratic candidate Michael Dukakis in the 1988 presidential campaign. The two most famous were the "Willie Horton" ad, which displayed grainy visuals of a black rapist and murderer while an unseen announcer blamed Dukakis for letting him free to rape again, and an ad which ridiculed Dukakis with visuals of him looking foolish while riding in a tank.

Attack ads can also backfire, however. If an ad is seen as going too far or being too personal the voters will turn against the party that put out the ad. One example of an attack ad backfiring was during the 1993 Canadian election when the Progressive Conservative Party of Canada attacked Liberal Party of Canada candidate Jean Chrétien by mocking his facial deformity. Outrage followed and the PCs were hurt badly in the polls.

Campaigns often establish or support front groups to run attack ads to deflect the criticism that comes from running them.

See also: Negative campaigning