The riding of Lac-Saint-Jean is usually a lost cause for the Liberal Party of Canada, which has won only a single time in the heartland of Quebec nationalism since 1958.

Prime Minister Justin Trudeau and his team are nonetheless working the area heavily ahead of a looming by-election, feeling an upset in the previously Conservative riding is within their grasp.

Mr. Trudeau spent two days in late July boosting support for his party in the region, including attending a large street festival in the city of Roberval where his personal popularity was on full display.

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The Liberals are planning another show of strength in the riding, with their Quebec team gathering on Aug. 30 and 31 in the city of Alma for their traditional summer caucus meeting. Party officials, who spoke on condition of anonymity, confirmed they deliberately picked the meeting spot as part of a charm offensive in the traditionally hostile territory.

While the Liberal nomination and the date of the by-election have yet to be announced, Mr. Trudeau is expected to move quickly to capitalize on his party's momentum in Quebec. A Liberal victory would be significant, as incumbents have won all previous by-elections in the current Parliament.

Pollster Jean-Marc Léger said the Liberals are up by more than 10 points in Quebec since the 2015 election, when they took a surprising haul of 40 out of 78 seats. In that context, he said they would be smart to strike as early as possible in Lac-Saint-Jean, even if the Liberal candidate finished well behind the Conservative Party and the NDP two years ago.

"In a riding like Lac-Saint-Jean, things are becoming tight. There are few seats in Quebec that are not within the reach of the Liberal Party right now," Mr. Léger said. "While Mr. Trudeau's popularity has gone down in the rest of Canada, it is still at its peak in Quebec."

Since a surprise Liberal victory in 1980, the riding currently known as Lac-Saint-Jean voted twice for Brian Mulroney's Progressive Conservatives, five times for the Bloc Québécois and four times for Stephen Harper's Conservatives.

The Conservatives are bracing for a fight to hold on to the riding, which they took over from the Bloc Québécois in 2007 when former Roberval mayor Denis Lebel made the jump into federal politics. Mr. Lebel, who became a cabinet minister and his party's Quebec lieutenant, won three more elections and proved to be a key player in the Harper government.

However, he announced in June that he was leaving politics and joining the Québec Forest Industry Council. The vacancy will prove a key test for the major federal parties in Quebec.

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"This is a seat we absolutely want to keep, although we know it won't be easy," said Conservative MP Alain Rayes, who is Conservative Leader Andrew Scheer's new Quebec lieutenant. "This is a test as much for the Liberals as for us as Conservatives. This by-election will give us a sense of what will happen in the next election."

The Conservatives have yet to nominate a candidate, but they already know on which issues they plan to fight the Liberals: the ongoing softwood dispute with the United States, the recent influx of asylum seekers in Quebec, agricultural policy and the controversial decision to legalize marijuana by next July.

"People are really concerned about the legislation [to legalize marijuana] that Mr. Trudeau is pushing through rapidly," said Mr. Rayes, who was in the riding with Mr. Scheer in late July. "People here are not at ease with that policy."

According to Liberal officials and would-be Liberal candidates, two elements are playing in their favour at this point: Mr. Trudeau's personal appeal, and the desire of Lac-Saint-Jean voters to have an MP on the government benches.

"I share many ideas with Mr. Trudeau, and there is now a good opportunity to serve the public on the side that is in power," said Richard Hébert, the mayor of Dolbeau-Mistassini who is already campaigning for the Liberal nomination. "When a party is in power, it allows people to get their message across, and voters here see in me someone who would have a strong voice in Ottawa."

Mr. Trudeau spent two days in the riding last month where he met up with Quebec Premier Philippe Couillard, who represents the area in the National Assembly. Mr. Trudeau's personal popularity was obvious as he walked through the crowd of thousands at an open-air supper, shaking hands and snapping selfies for nearly two hours.

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The following day, Mr. Trudeau played host to a roundtable with businesspeople and union officials, looking for ways to boost the economy in a region that is dependent on natural resources.

"We talked about the challenges facing Alma and the region," Mr. Trudeau told local reporters after the meeting. "I learned many things."

The NDP finished in second place in the riding in the 2011 and 2015 general elections. However, the party in the middle of a leadership race, which stands to hurt it on voting day.

The Bloc is also struggling in public-opinion polls under the leadership of Martine Ouellet, a former Parti Québécois leadership candidate who is currently sitting in Quebec's National Assembly as an independent.

"For the Bloc and the NDP, their odds are not very good in the riding at this point," Mr. Léger said.