Alan DeSousa, Saint-Laurent's long-serving borough mayor, won't be allowed to seek the Liberal nomination in the upcoming federal byelection, and the party is refusing to tell him why.

DeSousa's campaign for the nomination, launched earlier this month, came to an abrupt halt with a letter from the Liberal Party which informed him he was not considered a "Qualified Nomination Contestant."

He was hoping to take over the seat left vacant by Stéphane Dion, whose old riding of Saint-Laurent covers the same district DeSousa has represented in municipal politics for close to three decades.

​No reason is given in the letter, which DeSousa shared with reporters on Tuesday. The Liberal Party refused to explain its decision when contacted by CBC News, citing "confidentiality."

'Let's be very clear, the whole process is kind of opaque.' - Saint-Laurent borough mayor Alan DeSousa

Speaking outside the Saint-Laurent borough offices, DeSousa vowed to challenge the decision.

He warned the party it was underestimating his standing in the community by attempting to sideline his candidacy.

"The party has to wake up and smell the coffee," he said. "Let's be very clear, the whole process is kind of opaque."

Yolande James becomes front-runner

​The Saint-Laurent riding is considered a Liberal stronghold. With DeSousa out of the nomination race, the clear front-runner is Yolande James, a former provincial Liberal cabinet minister who says she was courted to run by the Grits.

James is 39 and was a regular contributor to Radio-Canada's popular political talk show Le Club Des Ex. (She was also a political columnist for CBC Montreal.)

Along with her ties to the provincial Liberals, James also has connections to their federal counterparts. One of her former chiefs of staff now works in the prime minister's office.

DeSousa, a chartered accountant by training, is in his late 50s and only joined the Liberal Party last month. He was a key member of former Montreal mayor Gérald Tremblay's administration before it collapsed in scandal in 2013.

Former provincial Liberal cabinet minister Yolande James is also seeking the nomination for the federal Liberals in Saint-Laurent. (CBC)

On Tuesday, DeSousa stood by his lengthy record of public service, saying he was mystified by the Liberal Party's rejection of his candidacy.

"I have nothing to hide," he said. "I have been in public office for 31 years.... At no point have I had an ethical lapse."

Without mentioning James by name, he acknowledged that his removal from the race favoured one of his opponents in particular.

"You can read between the lines," he said.

A spokesperson for the James campaign would only say the developments would not change her strategy.

The Saint-Laurent byelection is one of five that will be held around the country on April 3. Members of the local Liberal riding associations get to vote on who will represent the party.

No date has yet been set for the Saint Laurent nomination meeting. Along with James, Marwah Rizqy, a tax law professor, is also seeking the candidacy.

Appeal to Trudeau

The controversy over the Saint-Laurent nomination comes as another Liberal hopeful in Ontario is accusing the party of working against her nomination campaign.

Juanita Nathan, a local school board trustee seeking the Liberal nomination in Markham-Thornhill, said the party arbitrarily declared some 2,000 new members that she signed up to be ineligible to vote in the nomination contest.

The move, she said, was engineered to benefit one of her rivals for the nomination, Mary Ng, a senior staffer to Prime Minister Justin Trudeau.

Early in his leadership of the party, Trudeau pledged to have open nomination races in all 338 ridings in the country, putting an end to the practice of allowing star candidates to skirt local nomination battles.

DeSousa sought to remind Trudeau of that promise on Tuesday.

"I am appealing to the prime minister today to make sure that the promise he made is kept," DeSousa said. "I believe it is a question of local democracy and local people."