Yolande James started her campaign for the Liberal nomination in the Montreal stronghold of St-Laurent with a number of advantages: ministerial experience in Quebec City, name recognition as a television pundit, and friends in high places.

Ms. James now enjoys the added benefit of an easier road to victory after the Liberal Party of Canada rejected the candidacy of her main opponent, long-time municipal politician Alan DeSousa, for unknown reasons.

Mr. DeSousa was expected to be a strong contender in the heated nomination battle in the multiethnic riding. While he is not as well known at the provincial level, Mr. DeSousa had the advantages of living in the riding and representing the same territory as mayor of the borough of St-Laurent for the past 16 years.

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In a race in which local organization and the ability to sell party memberships were key, Mr. DeSousa was getting ready to mount a serious bid.

Late on Sunday, however, a letter from the group that vets potential candidates' applications informed him that his name would not be on the ballot at the Liberal nomination meeting. Mr. DeSousa, who publicly revealed his ouster on Tuesday, said the party did not provide any explanation in the terse letter.

"The Green Light Committee is not recommending you to be a qualified nomination contestant," chair Suh Kim told Mr. DeSousa in the letter. "I appreciate that this news will come as a disappointment to you and your campaign team."

The decision leaves two candidates in the race: Ms. James, and a professor of fiscal law at the University of Sherbrooke, Marwah Rizqy, who ran for the party in another riding in the 2015 general election.

A former provincial minister of immigration, the 39-year-old Ms. James has a number of allies in the federal Liberal hierarchy. In particular, two of her former provincial staffers work in the Prime Minister's Office, and another one is the chief of staff to the federal Minister of Health.

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In an interview last week, Ms. James rejected the notion that the nomination would be rigged in her favour.

"What I know is that there is a nomination process … and I'm in that process, just like everyone else," she said. "It's going to be contested, there is no doubt about it, and I expect it to be tough. But I'm pretty tough, too."

Mr. DeSousa decried the fact that members will not be allowed to vote for him if they so wish, adding that the Liberal Party's decision would clearly favour Ms. James.

"You don't have to be a rocket scientist to figure out that if my candidacy is not on the ballot, clearly it will [benefit] in all likelihood a candidate who might well be the chosen one," he said in an interview. "If I'm not there, you don't need a crystal ball to figure it out."

St-Laurent is a traditional Liberal stronghold, and a victory in the nomination race almost guarantees a seat in the House of Commons. Former cabinet minister and party leader Stéphane Dion, who retired from politics in January, held the riding for 21 years.

The Liberal Party had already been accused of favouring the candidacy of senior Liberal staffer Mary Ng in a parallel by-election in Markham-Thornhill.

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In that riding, the party announced on Feb. 20 that the period to sign up new members to vote on the nomination had been retroactively cut off at Feb. 14, irking rival candidates. Ms. Ng is on leave from her position as director of appointments in the PMO.

Liberal members will choose their candidate in Markham-Thornhill on March 4, while no date for the nomination in St-Laurent has been set. The by-elections in both ridings will be held on April 3, in addition to two in Alberta and one in Ottawa-Vanier.

Liberal spokesman Braeden Caley refused to explain the party's decision on Mr. DeSousa.

"The Liberal Party of Canada is committed to an open nomination for the Saint-Laurent by-election, and thousands of Saint-Laurent residents have signed up to be registered Liberals. For reasons of confidentiality, the party cannot discuss the details of a particular file," he said.

Through a spokeswoman, Ms. James refused to comment on "the party's decisions."