Canadians who say Conservative attack ads against Liberal Leader Justin Trudeau increase their desire to vote Liberal more than double those who say the ads inspire them to vote for the Conservatives, a new poll suggests.

One half of the randomly selected Canadian adults who took part in a Forum Research poll earlier this month said they had seen the television ad in which Mr. Trudeau is filmed while taking off his shirt at a fundraiser and a voiceover says "he's in over his head."

A little more than a third said they had seen another ad that the Liberals put out in response in which Mr. Trudeau sits on a schoolroom desk and says "together we will build a better country."

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Of those who had seen the ad, half said it will make them more likely to vote Liberal. That compares with the 19 per cent who said it makes them more likely to vote Conservative.

Even among Conservative supporters, one in 10 who had seen the ad said it makes them more likely to vote Liberal.

On the other hand, half of respondents who had seen the pro-Trudeau ad said it made them more likely to vote Liberal while 20 per cent said it will prompt them to vote Conservative and 15 per cent said it will push them towards the NDP.

Respondents were not given the option to say their views were unchanged.

The interactive voice response telephone survey of 1,779 Canadians was conducted on May 21 and 22, as the scandal in the Senate was unfolding. It is expected to accurately reflect the views of the population at large within 2 percentage points 19 times in 20.

Most people instinctively say they dislike negative advertising. And an aversion to an ad does not necessarily translate to the ballot box.

That's because the ads work on both the conscious and sub-conscious levels and are intended to create doubts about their target over the long term.

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But Lorne Bozinoff, the president of Forum Research, said these ads seem to have been less effective than their predecessors – possibly because Canadians know that previous Liberal leaders were taken down the same way and were expecting this round of advertising against Mr. Trudeau.

"People kind of knew what the playbook was," said Dr. Bozinoff. "I don't know if advertising can work when everyone looks at the playbook."

In addition, he said, the same poll suggested that the Liberals would have won a majority if an election had been held this week. Of those surveyed, 44 per cent said they supported the Liberals, 27 per cent said they supported the Conservatives and 20 per cent said they supported the New Democrats.

If they had any effect, the ads "seemed to have moved New Democrats into the Liberal column. That is the last thing the Tories want," said Dr. Bozinoff. "They want to split the opposition. they don't want the opposition to coalesce."

(Read more coverage of Canadian political polls)