Bloc Québécois Leader Gilles Duceppe says he'd be in favour of invoking the notwithstanding clause in the Charter of Rights and Freedoms to buttress legislation banning niqabs at citizenship ceremonies.

"I'd be for introducing the clause if ever such a law was found to contravene the charter," he said Tuesday.

The constitutional measure was put in place "precisely for this kind of situation," he added.

When the Conservatives tabled legislation last June to ban the wearing of niqabs at citizenship ceremonies, they said it respected the Charter of Rights and Freedoms.

That legislation ultimately died on the order paper after the federal election was called, but the Conservatives have promised — if re-elected — they would introduce and adopt legislation banning face coverings during citizenship oaths within the first 100 days.

A Federal Court of Appeal ruled last week that a woman could wear a niqab while swearing the oath.

The federal government is currently seeking a stay of that court decision.

'Politics of fear and division'

Duceppe's position comes after the sovereigntist party faced criticism for an online ad it posted, which showed oil leaking from a pipeline then morphing into a niqab.

In the ad, released last Friday, the Bloc warns a new pipeline is coming "even if we don't want it," if Tom Mulcair's NDP wins the election.

The ad also says Mulcair has no problem with a niqab being worn during a citizenship ceremony.

It is not the first time the party has tried to make the niqab a wedge issue for the NDP with Quebec voters. In a similar campaign ad, released in February, a narrator asked in French: "Should you have to hide your face to vote NDP?"

This advertisement for the Bloc Québécois asks whether it's necessary to hide one's face in order to vote for the NDP, referring to NDP Leader Tom Mulcair's view on the wearing of niqabs at citizenship ceremonies. (BLOC.org)

The text is superimposed on an image of the House of Commons through the eyeholes of a black niqab.

The Bloc is trying to claw its way back into the race in Quebec — where the New Democrats are ascendant — by painting Mulcair as out of touch with Quebecers on issues of religious accommodation.

Mulcair, for his part, said Monday that niqabs should be allowed at the public citizenship ceremony, but that women should be required to identify themselves without the veil at some point in the process.

NDP Leader Tom Mulcair says the courts have already ruled about niqabs being allowed at citizenship ceremonies. 1:23

"The courts have spoken in this case, and I think that everybody in Canada has to understand that you do indeed have to identify yourself, and I agree with that.

"But I'm not about the politics of fear and division. Mr. Harper is going to always go after that. If he senses that there's something there, that can divide Canadians one against the other, he'll do it. He talks about 'old stock' Canadians. That's a code word. He knows what he's doing when he does that."

Citizenship and Immigration Minister Chris Alexander said the government would appeal the Federal Court of Appeal ruling that would allow Zunera Ishaq to wear a niqab while swearing the oath of citizenship.

The government said it would ask for the ruling to be deferred while the issue is before the Supreme Court.

Should that happen, it is unlikely Ishaq would become a citizen in time to vote in the Oct. 19 federal election.