Prime Minister Justin Trudeau is facing a court challenge aimed at forcing him to name someone from Atlantic Canada as the next judge of the Supreme Court of Canada. The challenge pits a tradition of regional diversity that is as old as the court against the government's search for increased racial diversity – and possibly the first indigenous judge on the country's most powerful court.

The Atlantic Provinces Trial Lawyers Association applied to Nova Scotia Supreme Court on Monday for a declaration that the "Prime Minister's proposed departure from the constitutional convention of regional representation on the Supreme Court of Canada" requires a constitutional amendment and the unanimous consent of all provinces.

The court has a vacancy because Justice Thomas Cromwell of Nova Scotia retired on Sept. 1.

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Related: Newfoundland to remain without judge on Supreme Court

"If we lose our connection to the national institutions, from the perspective of Atlantic Canadians, that would further marginalize them," Halifax lawyer Raymond Wagner, whose name is on the court filing, told The Globe and Mail. The trial lawyers association represents plaintiffs with medical malpractice or personal injury claims.

Federal Justice Minister Jody Wilson-Raybould said in an e-mailed statement to The Globe that "regional representation is important and will be considered in the appointments process," and that the short list of three to five candidates to be given to the Prime Minister by an independent advisory board later this week will include candidates from Atlantic Canada.

The Atlantic lawyers' group brought the case after Mr. Trudeau introduced a self-nominating process for Supreme Court judges, and Ms. Wilson-Raybould told a government committee that applicants will be considered from inside and outside Atlantic Canada.

The court challenge raises the stakes for the Liberal government in the appointment. Even if it announces a high-profile First Nations appointment, the government could bump up against the tradition of regional representation – with roots so old, it may be protected by the Canadian constitution. And that could mean the Liberals pay a political price in Atlantic Canada, where they hold all 32 federal seats. And the actual appointee could be caught up in a legal or political quagmire.

Quebec is the only province guaranteed in written provisions of the Supreme Court Act three positions on the court. By convention, Ontario also has three, the Western provinces have two and Atlantic Canada one. From the Supreme Court's inception in 1875, it has always had a judge from Atlantic Canada.

The case has echoes of Toronto lawyer Rocco Galati's 2013 constitutional challenge to Federal Court Justice Marc Nadon's appointment to a Quebec vacancy on the Supreme Court, while Stephen Harper was prime minister. Justice Nadon was the first appointee to be ruled legally unqualified by the Supreme Court. And the court was left one judge short for 10 months, during the legal battle and its aftermath.

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One important difference between the two cases is that in the current one, the government has not yet announced its choice of a judge.

"It's very important for our organization that we not be seen as against the principle of diversity or a particular individual by name. That's why we brought it before the naming took place," Mr. Wagner said. If the court ignores the application, the organization will meet again once the government names a judge to decide whether to file another application, he added.

Multiple Liberal sources have told The Globe that the government is keen on finding a minority judge to replace Mr. Cromwell. Mr. Trudeau's insistence on functional bilingualism as a new requirement adds to the difficulty of finding a qualified minority judge in Atlantic Canada.

But does the tradition of regional representation amount to a constitutional convention? "Oh yes, definitely," Peter Russell, a political science professor emeritus at the University of Toronto, told The Globe. "An important part of our Constitution is to have the highest court in the land which interprets the Constitution have legitimacy in the various regions of Canada."