Most of today’s attack ads are mild by historical standards. Canada's first Prime Minister,  John A. Macdonald, had to contend with cartoons instructing his followers how to solicit bribes. In Britain, Disraeli faced attacks with anti-Semitic overtones. But what hasn't changed over the years is that, by emphasizing the personal rather than the policy, parties count on attack ads to influence voters. Then again, they might backfire. As Bernard Gauthier, head of the Ottawa public relations firm Delta Media, told CTV’s Canada AM on March 25, attack ads run the risk of alienating voters if they are perceived as "hitting below the belt." Yet the major political parties are prepared to take that risk.

"What's interesting is that when you start off in an election campaign you never know where that line is: where that belt is that you can't hit below," Gauthier said. As we’ve seen in past campaigns, voters will react strongly when the line is crossed.

For Canadians, the best-known such incident was the Conservative ads in 1993 that appeared to mock Jean Chretien’s Bell’s palsy. Outrage followed and the PC Party was hurt badly in the polls. There was similar backlash in 2006 to a Liberal Party attack ad (never aired, but leaked to the media) that suggested Stephen Harper would use the armed forces to police major cities. Its effect was to diminish the believability of the Liberal's other ads.

The impact of less sensational ads is more difficult to evaluate. People say they don’t like ads that attack leaders, yet sometimes they respond to them. Much depends on whether viewers have strong politics of their own, in which case the perception of an unfair attack or hidden agenda will never be far behind.

Non-party organizations that place ads for third parties often discover that politically informed people are very adept at identifying agendas in even the most neutral advertising. I have placed a number of third-party ads on behalf of several organizations over the years and I can’t recall a single instance where there weren’t complaints about “close ties to the Conservatives,” “obvious Liberal shill” or “the NDP must have written it.” In one case, the critics were talking about the same ad.

In politics, people see what they want to see, particularly if they are politically involved on a partisan basis. That doesn’t mean advertising is ineffective. But it does mean that the influence is likely to be strongest on people who vote but do not have strong political leanings, or are undecided about particular issues in one specific election. Those who are already strongly committed tend to see ads as affirming their opinions or for proof that the other side is wrong.

Most pundits are predicting more attack ads this time around, possibly because there are few strong issues for candidates to discuss. But whether theses ads will affect the outcome is another question altogether.