The Canadian political right is facing its biggest rift since it unified behind the Conservative Party of Canada 15 years ago, with Quebec MP Maxime Bernier vowing to create a new party that threatens to split the movement in the next federal election.

On the first day of the Conservative policy convention in Halifax, the former leadership contender announced he was leaving the CPC and unleashed a scathing attack against Conservative Leader Andrew Scheer, who narrowly beat him in last year’s leadership race.

Standing by himself at a news conference in Ottawa, Mr. Bernier accused Mr. Scheer of being a “moderate” who depends on polls and focus groups to make policy choices.

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“I have come to realize over the past year that this party is too intellectually and morally corrupt to be reformed," Mr. Bernier said, vowing that his new party will fight for “more freedom, less government.”

Opinion: The Bernier headache is just beginning

“I am now convinced that what we will get if Andrew Scheer becomes prime minister is just a more moderate version of the disastrous Trudeau government.”

No MP or riding president has so far followed Mr. Bernier to the unnamed political entity that is not yet registered with Elections Canada.

Still, the announcement upstaged the start of the Conservative convention, where Mr. Scheer wanted to present the image of a principled party ready to fight the Liberals on balanced budgets and tax cuts in the next federal election.

“I have always challenged [Mr. Bernier] to put personal ambition aside and to concentrate on the common ground that all conservatives can rally around,” Mr. Scheer told reporters. “Today it is clear that Mr. Bernier decided to help [Prime Minister] Justin Trudeau rather than his caucus colleagues and our team.”

The remarkable split between the former leadership rivals caps off more than a year of tension between Mr. Bernier and Mr. Scheer that began soon after the May, 2017, race to replace Stephen Harper at the helm of the party.

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In a chapter of his political memoir released in advance to The Globe and Mail this spring, Mr. Bernier, who opposes supply management which the Conservative Party supports, accused Mr. Scheer of winning the leadership because of “fake Conservatives” from the Quebec dairy lobby. Mr. Bernier was stripped of his critic portfolio after he reposted the chapter on his personal website despite his promise to caucus to shelve the book.

Emerging from a closed-door caucus meeting on Thursday, Conservative MPs said they were united behind Mr. Scheer.

Toronto-area MP Peter Kent said Mr. Bernier “deliberately planned today to throw a hand grenade to attempt to sabotage this convention for his own personal ambition."

Alberta MP Shannon Stubbs, the party’s natural resources critic, said Mr. Bernier’s decision to leave the party as 3,000 delegates discuss and debate policy shows that he’s not interested in beating the Liberals in the next election. “It is a slap in the face to the many Conservative members who supported him,” she said.

Mr. Scheer’s national campaign manager, Hamish Marshall, said the party is focused on building its team for the 2019 election.

“I don’t see this as anything more than a speed bump on the road to victory in 2019,” Mr. Marshall told The Globe. “Building a new party is extremely difficult and takes a lot of time and effort. And it’s one thing to get a lot of Twitter followers, it’s another thing to get a lot of votes.”

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The new party stands to split the political right across the country in the 2019 federal election, with some Conservatives fearing the consequences of a divided movement.

“It is clear that Max never accepted the result of the leadership vote and seeks only to divide Conservatives," Mr. Harper tweeted. "His decision today allows the Conservative Party of Canada to move forward united behind our Leader Andrew Scheer.”

Open this photo in gallery Conservative leader Andrew Scheer speaks to reporters at the Party's national convention in Halifax on Thursday, August 23, 2018. Darren Calabrese/The Canadian Press

One of Mr. Harper’s former senior advisers, Rachel Curran, predicted that the Liberals have “secured an easy win in 2019 despite a mostly terrible summer” because of the turmoil on the right. Still, Jason Kenney, a former colleague of Mr. Bernier’s in Ottawa and now the Leader of the United Conservative Party in Alberta, said the plan to create a new party was “a joke.”

“Conservatives need to be united to win. I think they are united and one man’s ego is not going to change that,” Mr. Kenney said.

Over the past two weeks, Mr. Bernier has been tweeting about diversity issues and what he called Mr. Trudeau’s “extreme multiculturalism.” His remarks led Mr. Scheer to accuse him of engaging in “identity politics” and to publicly distance the party from Mr. Bernier’s views.

Mr. Bernier said his decision to create a new party came after a private meeting with Mr. Scheer nine days earlier, in which it became clear that he was unable to advance his policy ideas within the party.

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“My leader told me and every Canadian that I didn’t have any influence in the party,” he said.

Mr. Bernier’s departure from the party was the talk of the Conservative convention among party members on Thursday.

“I was disappointed that he made that decision. I was hoping that he’d stay in the caucus. A lot of people in the party support his views,” said Devon Long, a delegate from Niagara Falls.

A long-time Bernier supporter, Clinton Desveaux, said he’s interested in joining Mr. Bernier’s new party.

“If he starts a new party and it’s based on the policies and ideas that he campaigned on – ending supply management, stopping corporate welfare, understanding what’s happening at the border, all of these issues – I would definitely support him,” said Mr. Desveaux, who hails from Nova Scotia and was handing out “End supply management” buttons outside of the convention centre in downtown Halifax.