An Ottawa-area Conservative MP says Canada's largest union of federal public servants shouldn't have endorsed separatist parties during the Quebec election and is calling for new rules allowing members to opt out of paying dues.

The proposal from MP Pierre Poilievre is similar to "right-to-work" legislation that has been adopted by more than 20 U.S. states, provoking heated debate and resistance from unions.

"I cannot imagine how it could possibly be in the interests of a Canadian public servant for the union to back a separatist party," Mr. Poilievre said in an interview. "And yet that is precisely what PSAC has done."

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A day before Tuesday's Quebec election, the National Capital Region branch of the Public Service Alliance of Canada announced the results of its assessment of the Quebec parties "on the basis of their positions on workers' and citizens' rights, public services and unions."

Based on that criteria, it ranked the Parti Québécois first, Québec Solidaire (which also supports a sovereign Quebec) second, followed by the Liberal Party and the Coalition Avenir Québec.

The union said the parties' positions on national unity were not considered as part of the ranking. It was not the first time PSAC has endorsed sovereigntist parties, both provincially and federally.

Mr. Poilievre says he's heard directly from public servants who are not happy with how PSAC manages its union dues.

As the parliamentary secretary to the transport minister, Mr. Poilievre is not allowed to introduce private members' bills. However, he said he will campaign behind the scenes to see if another MP – or possibly the government – would consider legislation allowing federally regulated workers to opt out of union dues. He noted that the vast majority of unionized workers in Canada are provincially regulated and would not be affected by any federal change.

"We have the freedom to associate, not the obligation to associate," he said. "There's no other freedom that we consider to be an obligation. Freedom of religion is not an obligation to be religious.… People should be free to decide whether or not they want to be part of a union."

The principle of mandatory union dues in Canada dates back to the 1940s, when Supreme Court Justice Ivan Rand was appointed as an arbitrator to end a Ford strike in Windsor. The ensuing "Rand formula" has largely prevailed ever since, establishing the principle that employers must deduct union dues from all workers.

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University of Manitoba law professor Debra Parkes says a law as proposed by Mr. Poilievre would certainly face a legal challenge, but in her assessment of related cases, it's not clear how the courts would rule.

"It's a big piece of our labour law system because of the problem of free riders," she said, noting that workers who refused to pay union dues would still benefit from collective bargaining. "It's politically a very difficult issue and one that most governments haven't wanted to take on."

Robin Benson, the national president of PSAC, said in a statement that PSAC members have the opportunity to vote on how their dues are spent.

"Mr. Poilievre is raising this issue now to distract from the fact that he and his government have been cutting valuable public services and good jobs without being transparent to Canadians," she said.