Conservative leader Stephen Harper's line about "old-stock Canadians" during  the Thursday leaders' debate was a deliberate move to energize the Conservative base on an emotional topic, a pollster says.

Frank Graves, president of EKOS Research, says that kind of divisional tactic has been used successfully in the past.

"It's part of the deliberate strategy to sort Harper's constituency from the rest of the electorate," Graves told CBC Montreal's Daybreak. "It creates a sense of us versus others."

Graves describes Harper's comment as a "dog whistle": something meant to be heard by a target audience, but misheard or ignored by the rest.

Harper made the comment while addressing health care for immigrants and refugees. 

It creates a sense of us versus others - Frank Graves, president of EKOS Research

"The fact of the matter is we have not taken away health care from immigrants and refugees ... The only place we have refused it is for bogus refugee claimants who have been refused and turned down, we do not offer them a better health care plan than the ordinary Canadian can receive," Harper said during the debate.

"I think that's something that both new and existing and old-stock Canadians can agree with."

On Friday, Harper was asked at a news conference what he meant by that phrase, the prime minister did not clarify his intention, but repeated his position on health care for "genuine refugees". He then added:

"It's supported by Canadians who are themselves immigrants and also supported by the rest of us, Canadians who have been the descendents of immigrants for one or more generations."

Politicians have used the line before

It's not the first time "old stock" has been used by Canadian political leaders.

All my life that word has been used to say I'm not a real Canadian - Marlene Jennings, Former Quebec Liberal MP 

In a 2007 interview with a Montreal newspaper, Justin Trudeau said that he wondered if everyone in Quebec was part of a nation or just the "old-stock pioneers."

Liberal MP Stéphane Dion used the line before the Canadian Heritage Committee in 2014.

"If I'm fishing with a friend on a magnificent lake in the Laurentians ... and I see a small boat in the distance ... usually it's two middle-aged old-stock French-Canadians or English-Canadians," he said at the time.

A codeword for non-white Canadians

Former Quebec Liberal MP Marlene Jennings took offence to Harper's use of the phrase in the debate.

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Marlene Jennings, seen here in 2009, served as the MP for NDG-Lachine for 14 years before losing the last federal election. (Adrian Wyld/Canadian Press)

"All my life that word has been used to say I'm not a real Canadian," said Jennings, who was the first black woman from Quebec to be elected to Parliament.

"It's used against people who don't quite look like what some people think Canadians should look like," she told Daybreak.

Jennings said the word has been used as far back as the early 20th century, when large waves of immigrants from Eastern Europe came to Canada.