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Survey from Innovative Research suggests Liberal Leader Justin Trudeau may need to worry about the cumulative effect of rival parties’ ads against him.

After playing a pivotal role in the surge of Justin Trudeau's Liberals through most of the federal election campaign, television and radio advertising may be threatening their momentum in the race's final stretch.

An ongoing series of surveys by Innovative Research Group, gauging voters' reactions to parties' advertising during the campaign, has found many of the Liberals' ads more effective than those of their opponents. It is an unlikely coincidence that the positive responses to the Liberal ads – most notably one in which Mr. Trudeau directly rebutts the "not ready" charge against him, and another showing him going up a down escalator to make a point about Canadians' difficulty getting ahead under the Conservatives – preceded major gains for the Liberals in horse race polls.

But the most recent such survey, which in the first week of October tested a dozen campaign ads with 2,400 randomly selected voters participating in an online panel, found that recent Liberal ads were having a more modest impact than previous ones. At the same time, most of the other parties' ads appeared to be chipping away at Liberal support.

Of four Liberal ads tested in this round, only one – a television spot featuring that party's high-profile candidates, interspersed with clips of Mr. Trudeau speaking at a rally – seemed to work well. Among respondents asked a series of questions before and after viewing that ad, it caused a significant bump in Liberal support as well as perceptions of Mr. Trudeau relative to the other leaders.

A Liberal ad in which Mr. Trudeau promises a break from "more of the same" amid repetitive images of Conservative Leader Stephen Harper – which appeared, based on recognition of it, to be in heavier rotation – had no statistically significant impact on respondents' voting intentions and only a small one on attitudes toward the leaders. A radio ad in which Mr. Trudeau presents himself as more of a change agent than the NDP's Tom Mulcair helped the Liberal Leader on the question of who will stand up for the middle class, but its impact was otherwise negligible. And a French-language ad touting the Liberals' child-benefit promises (tested only on respondents from Quebec) did not move any numbers in a meaningful way.

None of the other parties' ads taking direct or indirect aim at the Liberals appeared to be overwhelmingly effective either. But the concern for Mr. Trudeau's party, according to Innovative Research managing director Greg Lyle, could be more cumulative, with their opponents "effectively chipping away" at Liberal support.

The most hard-edged of the recent attempts have been NDP radio ads, including one attacking Mr. Trudeau for accepting speaking fees while an MP. That spot elicited strong emotional reactions when tested, including some negative ones from NDP backers. But support for the Liberals went down by several points after respondents heard it.

The NDP's TV ads, while less aggressive than the radio ones, also showed some ability to hurt the Liberals. One in English, in which Mr. Mulcair makes the case for why his version of "change" would be more lasting than Mr. Trudeau's, drove up perceptions of the NDP Leader at Mr. Trudeau's expense. And a French spot aimed at showing nationwide NDP momentum increased support among Quebec respondents at the Liberals' expense.

Meanwhile, a Conservative ad that lists Mr. Harper's policies alongside Mr. Trudeau's (or at least the Tories' take on them) also drove down Liberal support by a few points among those who saw it – not a huge hit compared with what that party's ads did to the Liberals before the campaign officially began, but still potentially relevant as part of the bigger picture.

(Ads are rotated in Innovative Research's surveys so each participant focuses primarily on one of them. Detailed methodology and results are available at

It may be that by this point in the campaign, voters' opinions have sufficiently hardened – and, in this case, the Liberals' momentum so taken hold – that even relatively effective ads don't really change much. And to the extent they do still matter, one put out this week by the Liberals, which features Mr. Trudeau speaking at a massive rally and has not yet been tested by Innovative Research, could prove more helpful to his cause than his opponents' efforts are hurtful.

But as suggested by the Liberal gains earlier in the race, there can be a slight delay before most polls reflect advertising's effect. That will be worth keeping in mind, if Mr. Trudeau's momentum stalls in the campaign's final days.