Patricia Chartier, a staffer for Quebec MP Philip Toone, confirmed to The Canadian Press she will be the Quebec solidaire candidate in the provincial riding of Bonaventure.
The Parti Quebecois, Quebec's biggest sovereigntist party, used Twitter to attack the apparent contradiction in Chartier's political loyalties.
"Federalist at work and sovereigntist on the campaign?," Pascal Berube, PQ member of the national assembly, wrote Friday.
Berube questioned in a later post if Toone would openly throw his support to his political attache running for a sovereigntist party.
The NDP and Toone did not return calls placed Friday afternoon.
Contacted at one of Toone's constituency offices, Chartier declined to discuss her Quebec solidaire candidacy while at work. She promised to speak further over the weekend.
It won't be the first time Chartier has run for Quebec solidaire — she finished last under the party's banner in 2008, more than 10,000 votes behind Liberal Nathalie Normandeau, who announced this week she was leaving politics.
The political allegiances of many NDP MPs from Quebec have come under scrutiny since 59 of them were swept to power in the last federal election.
That scrutiny only amplified after it was revealed the party's interim leader, Nycole Turmel, had held memberships in both the Bloc Quebecois and Quebec solidaire.
Some media outlets even distributed questionnaires to the NDP's Quebec MPs in an effort to determine how many have been affiliated with sovereigntist parties in the past.
Quebec solidaire is a bit player on the provincial scene with only one member in the legislature and an estimated 5,000 members.
There is a clear overlap between its priorities — which include fighting poverty and promoting the environment —and those espoused by the NDP.
But Quebec solidaire's leadership is clear about one thing — its candidates are sovereigntists.
"Candidates have to be comfortable with that choice and not say the opposite," said Francoise David, the party's president and one of its principal spokespeople.
"I've known Ms. Chartier for many years... she is above all an activist for social justice. I didn't force her to undergo a test to see to what degree, from zero to 10, she was a sovereigntist."
She said Chartier will take an unpaid leave of absence from her NDP job when she begins to campaign for the byelection, whose date has yet to be set.
Chartier has worked for several years in the riding of Bonaventure, which covers much of the same territory as Toone's federal riding of Gaspesie–Iles-de-la-Madeleine.
According to David, Chartier considers her work for Toone as a chance to advance the progressive cause.
David dismissed the controversy over her candidacy as a "tempest in a teapot," one stemming from confusion in the rest of Canada over the nature of politics in the province.
"I can understand that for people from English Canada it can seem full of contradictions and grey areas, but that's the Quebec reality," David said.
"It is full of people who want to build a progressive Quebec and even in certain circumstances people can be sovereigntist and still vote NDP. I saw it myself May 2."
The NDP's surge in Quebec during the May election reduced the Bloc to a mere four seats in the House of Commons.
Analysts have suggested the NDP was able to draw support from voters who in the past identified with sovereigntist parties.
But progressive politics in Quebec has been so closely entwined with the sovereigntist movement that it can be difficult to differentiate between the two.
One of the province's leading political analysts warned the rest of Canada that distinguishing a federalist from a sovereigntist may not be as easy at it seems.
"The Quebec mainstream is in large part sovereigntist, it is part of our society, it is not a marginal option," said Christian Dufour, who teaches politics at Quebec's public administration school.
"At the same time, Quebecers are very ambivalent. We can have a mixture of political aspirations."