The Liberal leader says Harper's stance — which the Conservative party has enthusiastically embraced to rally support, raise money and pad its voter data base — is unworthy of a prime minister in such a diverse, multicultural country.
Harper sparked the criticism after vowing last week to appeal a court ruling that allowed a Muslim woman to take the citizenship oath without removing her niqab, a religious face-covering garment that leaves only the eyes exposed.
Harper said it's "offensive" to hide one's identity while joining "the Canadian family."
Citizenship and Immigration Minister Chris Alexander went further in a subsequent email to Conservative supporters, urging them to sign an online petition in support of Harper's remarks; he suggested Muslim women should not be allowed to take the oath while wearing a hijab, which covers the head but not the face.
"Canada's diversity is our great and unique strength," Trudeau said in an interview with The Canadian Press.
"We are the one country in the world that has figured out how to be strong, not in spite of our differences but because of them. So, the prime minister of this country has a responsibility to bring people together in this country, not to divide us by pandering to some people's fears."
Harper's approach "frays away the edges of our multicultural fabric ... (by) stoking and pandering to fears rather than allaying them," he added.
What's more, "it's unworthy of someone who is prime minister for all Canadians."
A spokesman for the prime minister declined to comment on Trudeau's criticism, saying there was nothing to add beyond what Harper said last week: "I believe, and I think most Canadians believe, that it is offensive that someone would hide their identity at the very moment where they are committing to join the Canadian family. This is a society that is transparent, open and where people are equal."
Harper made his comments in Quebec, where popular support for his government's tough-on-terrorism stance may be boosting Conservative party fortunes as it prepares for an election in eight months.
The momentum follows the government's decision last fall to join in air strikes against Islamic extremists in Iraq, the murders of two Canadian soldiers last October by two home-grown jihadist sympathizers and the introduction this month of sweeping new anti-terrorism measures.
In adding the issue of veiled citizenship oath-takers to the mix, Trudeau said Harper is taking a page out of the pre-election play book of former Quebec premier Pauline Marois, who introduced a charter of Quebec values that would have banned public servants from wearing any obvious religious symbols. Despite the charter's initial popularity, Marois' Parti Quebecois was ultimately trounced in last spring's provincial election.
"When former premier Marois tried to do what Mr. Harper is now doing, I pointed out that Quebecers are better than that and that's exactly what happened. So, I feel the same way about Canadians everywhere. We are a better, stronger people than Mr. Harper seems to think we are," Trudeau said.
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