Supreme Court annuls 2025 one-vote Liberal win in Montreal riding

The Supreme Court of Canada has annulled a one-vote win by Prime Minister Mark Carney’s Liberals in a Montreal-area riding in the 2025 federal election, a decision which will force a byelection and pushes the governing party further away from a majority in the House of Commons.

It was the first time since 1988 that the courts have stepped in to overturn an election result.

Liberal Tatiana Auguste beat Bloc Québécois incumbent Nathalie Sinclair-Desgagné in Terrebonne by one vote in last spring’s general election, but it quickly emerged that one BQ supporter had her mail-in ballot returned because of an addressing error by Elections Canada, an error Elections Canada admitted to at the time.

Sinclair-Desgagné challenged the election results in court after learning that BQ supporter Emmanuelle Bossé had her mail-in ballot returned to her own mailbox, due to an error on the postal code printed on a label supplied by Elections Canada.

Sinclair-Desgagné’s challenge was rejected by the Quebec Superior Court. She appealed to the Supreme Court, and on Friday the court overturned that ruling within a few hours of hearing oral arguments from lawyers for both the Liberals and the BQ.

The Supreme Court’s ruling on Friday sets in motion the normal process that occurs whenever a seat in the House of Commons is declared vacant.

“The Liberal Party of Canada has taken note of the Supreme Court’s decision,” said Liberal Party spokesperson Matteo Rossi.

“We respect the ruling and the institutions that reinforce the strong confidence that Canadians have in the integrity of our democratic process. Tatiana Auguste ran a strong campaign, and whenever the byelection gets called, we are ready to engage with voters in Terrebonne with our plan to build a stronger, more resilient, and more prosperous future.”

Sinclair-Desgagné, the Bloc Québécois candidate, let out a long sigh of relief shortly after she heard the ruling, announced from the bench following the hearing earlier on Friday.

“Now that the seat is vacant, we are demanding that the byelection be called without delay, so that Terrebonne residents can go the polls rapidly,” said Suzanne Proulx, president of the Bloc.

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Sinclair-Desgagné was first elected in the 2021 general election, beating the Liberal candidate then by nearly 7,000 votes. Until Auguste’s one-vote win last spring, the Liberals had not won Terrebonne since 1980.

With Friday’s Supreme Court ruling, there are now three vacant seats in the House of Commons. The other two are in Toronto. It will be up to Prime Minister Mark Carney to decide when all three byelections will be held. Carney has some flexibility in terms of timing but all three byelections are expected to be completed by the end of the summer.

And there may soon a be a fourth vacancy in the House. Liberal Nate Erskine-Smith has indicated he will resign as MP for the Toronto riding of Beaches–East York as soon as Ontario Premier Doug Ford calls a byelection for the provincial riding of Scarborough Southwest. The MPP for Scarborough Southwest was New Democrat Doly Begum, but Begum switched parties to become the Liberal candidate for the federal riding of Scarborough Southwest, which became vacant when Bill Blair resigned as MP to become Canada’s High Commissioner to the United Kingdom.

Carney was in Tumbler Ridge, B.C., on Friday and did not comment on the Supreme Court ruling.

In any event, even if the Liberals lost all three or four byelections, their minority government would not be in danger. Losing any one of those seats would make it more difficult for the Liberals to gain a majority through, for example, floor-crossing opposition MPs.

—With files from The Canadian Press

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