Conservative MP Larry Brock on Thursday announced that he would resign his seat in the House of Commons days before the start of the fall sitting in September, setting the stage for at least five more byelections to be called by spring 2027.

Since the start of the 45th Parliament in the spring of 2025, there have already been four byelections held and three more are scheduled for Aug. 31. The Liberals will need to at least win one of those races to retain their razor-thin majority.

Brock, a two-term MP, said he was vacating his seat on Sept. 18 to return to his previous job as a Crown prosecutor.

The prime minister has to call a byelection between 11 and 180 days after a seat becomes vacant and formal byelection campaigns last between 36 and 50 days.

The resignation wasn’t entirely unexpected after Brock announced earlier in the summer that he was standing down as the Conservative justice critic.

“My commitment to community safety hasn’t wavered one bit, and getting back into the courthouse means I can keep being tough on crime, making sure real justice happens right here at home,” he said in a letter that was posted on X.

Brock’s midwestern Ontario riding is centred on the city of Brantford and had its boundaries reconfigured prior to the 2025 election. It will officially be known as Brantford-Brant South starting next month, one of several riding name changes made in the Liberal government’s elections bill that passed in June.

This latest departure means as many as five seats could be vacant when the House resumes sitting on Sept. 21.

Pollster Dan Arnold said the Liberals will want to “strike when the iron is hot” and call the byelections quickly to take advantage of Prime Minister Mark Carney’s popularity.

He said Justin Trudeau made a mistake of delaying several byelections and was forced to call them during the midst of the SNC-Lavalin scandal in early 2019.

“I think when you’re doing good in the polls, you want to bank that as quickly as you can,” said Arnold, who previously headed up the Liberal research campaign teams and now serves as the chief strategy officer of Pollara.

“My expectation is that these byelections probably get called as as quickly as can reasonably be done from the Liberal perspective, because I think they want to make sure that we don’t get into a position where these byelections drift into 2027, and suddenly Mark Carney is not popular anymore.”

Despite these concerns, Arnold said Conservative Leader Pierre Poilievre has the most at stake in the upcoming votes and could face serious challenges to his leadership if the party’s loses Brock’s seat.

He said the Conservatives had their worst byelection performance since 2015 in April, where the party’s vote share dropped by over 10 points in all three ridings.

While those votes were held in usually Conservative-averse seats in Toronto and Montreal, Arnold warned that a similar drop in support in Brantford-Brant South could cost the party the seat — something that would be difficult for Poilievre to dismiss, unlike the recent string of poor polling.

“Anytime when you lose a seat that just psychologically has a bigger impact. So, I think if they lost Brantford… I think the psychology of that would be akin to when the Liberals lost St. Paul’s,” he said referring to the Toronto seat the Conservatives won in June 2024 that marked the beginning of the end of Trudeau’s leadership of the Liberals. After that loss, Liberal MPs began calling for his ouster, and he was forced to step down in January 2025.

David Valentin, principal with pollster Liaison Strategies, agreed that Poilievre can’t afford to lose this seat and suggested Carney might call the vote quickly to take advantage of the Conservatives’ poor polling in Ontario.

“This is the kind of Ontario seat that the Conservatives have to win to be competitive. Period. So. if they lose or if they come just by a sliver to victory, this sends a really disquieting signal,” he said.

While the region has historically voted Conservative, Valentin said the party is consistently down by at least 10 points in Liaison’s Ontario polling and those “are not numbers that make it easy” to hold a riding like Brantford-Brant South.

Liaison’s latest federal poll had the Liberals leading the Conservatives 48 to 36 per cent in Ontario. In the 2025 election, the Liberals won 49 per cent of the vote in the province, compared to nearly 44 per cent for the Conservatives.

The race could come down to candidate selection, Valentin said, and the Conservatives will want to avoid the intra-party feud that erupted during the last byelection in the region back in 2023. That race in nearby Oxford was much closer than the previous election after the Conservatives appointed Poilievre-loyalist Arpan Khanna as their candidate instead of holding an open nomination race.

“There’s there’s going to be a lot of people who who consider this a real opportunity to go to to go to Parliament Hill, and winning this nomination is their best chance,” Valentin said.

“So are you going to see the Tories hold an open nomination, or will they [make] an appointment of some kind?”

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Even if the party holds Brantford, Arnold said the sheer number of byelections could make it challenging for Poilievre to write off multiple poor performances.

Including the latest wave of resignations, there could be as many as 11 byelections held this year. And those ridings are rather diverse. There are seats up for grabs in Toronto, Montreal and Vancouver, as well as rural Quebec, Saskatchewan and the aforementioned race in southern Ontario.

“When you lose what’s seen as a safe seat, that really gets people’s attention. But if they drop an average of five points across these byelections, that’s also concerning,” Arnold said.

“It’s less dramatic. It’s less visceral than losing a seat like Branford. But I think it would be legitimately cause for concern if you’re consistently going down in your vote share in urban centres, in rural centres, in Liberal seats, in Conservative seats, in Quebec, in other parts of the country. That starts to become more than just a one-off.”

The wave of resignations will start picking up steam this month.

Quebec Liberal MP Steven Guilbeault is expected to resign on Aug. 28, while Saskatchewan Conservative MP Cathay Wagantall said she will step down three days later.

Guilbeault said he was leaving politics to return to environmental advocacy because of his disappointment with the Liberal government’s support of a new West Coast oil pipeline and reversal of key climate policies.

Wagantall had already ruled out seeking re-election but announced in June that she would leave before the end of the current Parliament. No reason was provided for the change in plans.

Alexandre Boulerice is set to resign his Montreal seat in the coming weeks to run for a left-wing sovereigntist party in Quebec’s provincial election, which is expected to take place in October. Originally elected as a member of the New Democrats, Boulerice resigned from caucus to sit as an independent in the spring after announcing his plans to run provincially.

Simon-Pierre Savard-Tremblay is also leaving Parliament to run in Quebec’s provincial election. The former Bloc Quebecois MP resigned his seat in June but it wasn’t included in the Aug. 31 byelections.

The Liberals have won three of the four byelections held this Parliament, including picking up long-time Bloc Quebecois stronghold Terrbonne.

Liberal Tatiana Auguste was originally declared the winner by a single vote in the April 2025 election, but the Supreme Court invalidated the result because of a mailing error. She won the riding in a byelection this past spring by over 700 votes.

The Liberals held two of three seats that will be contested on Aug. 31.

Jonathan Wilkinson easily won re-election in North Vancouver-Capilano last year but resigned this past spring to become Canada’s ambassador to the European Union. Braeden Caley, who was mostly recently Carney’s deputy of chief staff, is running for the Liberals in the riding.

The seat is usually a two-party race between the Liberals and Conservatives, though the Carney government’s support of a new oil pipeline to the West Coast has some political watchers wondering if the NDP will enjoy a boost in support.

Before resigning earlier this summer, Nate Erskine-Smith had represented the central Toronto riding of Beaches-East York since 2015. He announced earlier this year that would resign to run for the Ontario Liberal leadership. He changed course on running for that position after losing a nomination race for the provincial byelection in Scarborough Southwest.

His longtime constituency staffer Tanveer Shahnawaz won the Liberal nomination race in July.

Pollsters describe the riding as one of Toronto’s most progressive and expect the NDP’s Shannon Devine to emerge as Shahnawaz’s main rival, even though the Conservatives finished second in 2025.

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A byelection was called in Chicoutimi—Le Fjord after Conservative MP Richard Martel resigned to accept an appointment to the Senate last month.

Martel won re-election in 2025 by nearly 1,500 votes in a close three-way race against the Bloc Quebecois and the Liberals.

Arnold said the riding’s status as a battleground means there’s less risk for Poilievre if the Conservatives lose, but that’s only if the party remains competitive.

“I don’t think anybody would be surprised if the Conservatives lose that seat, and I think it’d be hard to really fault Poilievre for that. When the popular incumbent leaves, you expect the vote to fall a little bit,” he said.

“Maybe it’s concerning that Martel decided to to leave to go to the Senate, and if they drop by more than a couple points [or] if they end up a distant third in that one, that would be a cause for concern in terms of their long term viability [in] Quebec.”

Liberal Denis Lemieux won the riding in 2015, but resigned two years later. Martel won the subsequent byelection in 2018.

The Bloc had held the seat from 2004 to 2011, and the NDP held it for a single term after wining it in the ‘Orange Wave’ election of 2011.

Cégep de Chicoutimi law professor Régis Gaudreault is running for the Conservatives, while the Bloc nominated Caroline Dubé, who previously headed up the office of neighbouring Bloc MP Mario Simard.

The Liberals are running dairy farmer Daniel Gobeil.

iPolitics will have liveblog coverage of the Aug. 31 byelections.

with a file from the Canadian Press