The Liberal Party is being accused of favouring certain candidates over others in two nomination contests in Ontario and Quebec, raising questions about preferential treatment before several critical byelections.

In one case, a Liberal candidate hopeful says she was approached by a rival candidate -- a former senior staffer in the Prime Minister’s Office -- who she says asked her to drop out.

In another case, a candidate with more than 10 years of local political experience in Montreal says he wasn’t given an explanation when he was dropped from the ballot.

In a statement to CTV News, the Liberal Party said details of the nominations are confidential.

“I don’t think it’s acceptable to have the candidacy of a person rejected so easily with no reasons given,” Alan DeSousa, a Montreal borough mayor for the past 17 years, told CTV News.

DeSousa was considered a strong contender in the race to represent the party in the St. Laurent riding, a Liberal stronghold that opened up after Stephane Dion left Parliament in January.

DeSousa says he was recently informed by a group that approves candidates for the nomination vote that his name wouldn’t be on the ballot. He says he was given no explanation why.

The decision leaves two candidates in the running: Yolande James, a former Liberal cabinet minister in Quebec, and law professor Marwah Rizqy.

“Some people say that there is already a preferred candidate in there,” DeSousa said.

Similar allegations have surfaced in Markham-Thornhill, a riding in the Greater Toronto Area recently vacated by John McCallum.

Nomination hopeful Juanita Nathan said her campaign was recently approached by representatives from candidate Mary Ng. According to Nathan, the rival team suggested that she abandon her bid and throw her support behind Ng, who does not live in the riding and recently took a leave from her job with the PMO.

“I don't see why I should step aside for a preferential candidate,” Nathan said.

Ng’s campaign has denied the claims and said that at no time did they expect or request Nathan to leave the race.

Prime Minister Justin Trudeau was in Calgary Wednesday stumping for his party ahead of two Alberta by-elections next month. Trudeau underscored his party’s commitment to building pipelines and accused the former Conservative government of not being able to get the job done.

“Calgarians are tired of being taken for grant by people who talk a good game, but, as you say around here, it's all hat and no cattle,” Trudeau said.

Conservative Interim Leader Rona Ambose accused the Liberals of playing favourites.

"It’s the same old Liberals," she said. "They pick their favourites and then people that have an interest in running find out that they have to step aside and headquarters tells them that."

Votes will be cast in ridings vacated by Stephen Harper, who briefly stayed on as MP for Calgary Heritage after the 2015 election, and Jason Kenney, who is running for leadership of Alberta’s Progressive Conservative Party.

The five byelections will be held April 3.

With a report from CTV’s Omar Sachedina