OTTAWA--NDP Leader Tom Mulcair was hit with a double whammy Thursday as one of his Quebec MPs bolted to the separatist Bloc Quebecois.

Claude Patry's defection simultaneously raised doubts about the NDP's ability to hang on to its newfound base in Quebec and gave fodder to federalist rivals to accuse the NDP of being a hotbed of crypto-separatists.

Patry blamed his abrupt exit on the NDP's controversial unity bill -- which proposes that a bare majority Yes vote would be sufficient to trigger negotiations on Quebec secession.

Patry, who voted for Quebec independence in the 1980 and 1995 referendums, maintained the bill interferes with the right of Quebecers to determine their own future.

"I will not get down on my knees," he said, using a turn of phrase commonly used by sovereigntists to refer to those who put the interests of Canada before Quebec's.

Mulcair retorted that "every single (NDP) candidate, including Mr. Patry" ran on a platform in 2011 that included the party's Sherbrooke Declaration, on which the recently introduced unity bill is based.

And it was because of Sherbrooke -- "which is about openness towards Quebec within the framework of Canadian federalism" -- that the NDP swept 59 of Quebec's 75 seats, he maintained.

"That's how Claude Patry got elected in Jonquiere-Alma," Mulcair told a hastily called news conference, calling on Patry to face a byelection in the riding, where the Bloc captured just 18 per cent of the vote in 2011.

Prime Minister Stephen Harper seized on the defection to score political points, without waiting to be asked about it during an event in Riviere-du-Loup, Que.

"This is an issue that has concerned us for some time, and does concern us: the ambiguity on Canadian unity that we have among some members of the NDP caucus in Quebec," he said.

Harper said New Democrat MPs have "many, many links" to the sovereigntist Quebec solidaire -- a "very strong" provincial party which currently holds two seats in the province's national assembly.

"This phenomenon with 'Bloc orange,' I think, should give everyone considerable pause, and I think what has happened today is really another example of this particular problem."

Liberal leadership front-runner Justin Trudeau piled on, arguing that Mulcair has fallen into the "trap" of pandering to hardline Quebec nationalists, only to discover they will never be satisfied with anything less than independence.

"This is the result -- and we've seen it before with Brian Mulroney and Lucien Bouchard," Trudeau said in a phone interview.

"When you make alliances with nationalists and hard nationalists for temporary electoral gain, there are real dangers and consequences."

Bouchard, a one-time sovereigntist, sparked a unity crisis in 1990 when he stormed out of former prime minister Mulroney's cabinet during constitutional wrangling aimed at gaining Quebec's signature on the Constitution. He went on to form the Bloc Quebecois, lead the 1995 referendum campaign that came within a hair of breaking up the country and eventually become Quebec premier.

The consequences of Patry's defection are considerably less dire, although Trudeau noted it has boosted the Bloc's depleted ranks in the House of Commons by 25 per cent -- from four to five MPs.

Still, he said it's "a perfect example" of how cosying up to hardline nationalists "leads to bringing up old wounds and highlighting divisions instead of working together."

"Pandering to the loud but small proportion of people who are still very much fervent sovereigntists is a mistake that Mr. Mulcair has stepped into with both feet."

Trudeau has accused Mulcair of pandering to nationalists with the NDP's proposed unity bill, which is meant to replace the Clarity Act, brought in by former Liberal prime minister Jean Chretien after the 1995 referendum.

The Clarity Act sets a higher standard for triggering secession negotiations, stipulating that an undefined "clear majority" of Quebecers must vote Yes to a clear question on independence.

Like Harper, Trudeau called Thursday on the NDP's Quebec MPs who've flirted with sovereignty in the past to "come clean" on their current beliefs.

Mulcair brushed off the attacks from rival federalists, insisting his party is a resolutely federalist, socially democratic party.

"I think it's good news for all of Canada that, for the first time in a generation, Quebecers voted majoritarily for a federalist party, and we're going to continue to fight hard for winning conditions for Canada within Quebec," he said, insisting his caucus is united behind Sherbrooke and the unity bill.

Mulcair noted that Patry voted just a year ago in favour of an NDP motion calling on any MP who defects to another party to seek the approval of his or her constituents in a byelection.

"We're obviously going to ask Mr. Patry to have the courage of his convictions and to step down and to face his electors because we're 100 per cent sure that we will maintain that riding if he has the courage to confront his electors."

Alexandre Boulerice, one of the NDP's Quebec MPs who's admitted supporting sovereignty in the past, said he's not worried any other caucus colleagues will follow Patry's lead.

"We are strong, we are together and, you know, sometimes you have a bad day. Today was a bad day for us," he said.

This is the third defection from the NDP since the May 2011 election vaulted the party into official Opposition status.

Shortly after Mulcair became leader last March, Northern Ontario MP Bruce Hyer left to sit as an independent in protest against the NDP's support for the now-defunct long-gun registry. Shortly before Mulcair's election, Quebec MP Lise St-Denis crossed over to the Liberals.

The NDP now has 100 seats, 57 of them in Quebec.