The NDP and Liberal Party's plans to change the federal electoral system without necessarily holding a referendum has Conservative critics accusing them of playing loose with the democratic process.

Pressure is growing to change Canada's first-past-the-post (FPTP) electoral system, in which candidates can win their ridings – and parties can form government – with fewer than 50 per cent of the votes. Both of Canada's major opposition parties say that if they win the Oct. 19 election under the current system, they will change the electoral process before the next ballot.

"It would be the last unfair election," New Democrat MP Peter Julian said in an interview.

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The Liberal Party officially came on board on Tuesday with the release of a proposal to study options and present legislation to change the process within 18 months of forming government.

"We are committed to ensuring that the 2015 election will be the last federal election using first past the post," Liberal Leader Justin Trudeau said.

The Conservative Party said it would fight to keep the current system, stating voters have already laid out their opposition to different forms of proportional representation, including in referendums in Ontario and British Columbia.

"Every time Canadians have voted on this, they chose to keep the current system. We will continue to respect the democratic will of Canadians," said Pierre Poilievre, the Minister for Democratic Reform.

David McLaughlin, a former Conservative staffer and public policy expert, said MPs and political parties have a stake in the electoral system and cannot be trusted to have the final word. He was involved in a planned electoral reform in New Brunswick that was abandoned after a change in government in 2006.

"A referendum is an absolute necessity to confer public legitimacy upon the outcome," he said.

A group that has lobbied federal parties to support proportional representation, Fair Vote Canada, acknowledged the need to consult Canadians on electoral reform, but stopped short of calling for a vote.

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"Absolutely, it should go to the people, but not necessarily through a referendum. We would prefer a citizens' assembly," executive director Kelly Carmichael said.

Mr. Trudeau has indicated his preference in the past for ranked ballots, in which the second, third or fourth choices of electors are taken into account until a candidate gets more than half of the votes. However, the Liberals are also opening the door to a form of proportional representation, in which voters still have a member of Parliament, but other MPs are elected based on their party's share of the overall vote.

Mr. Trudeau has rejected calls to put his government's proposal to a vote, saying he would hold a public consultation led by a committee of MPs.

"We are fully committed to serious, in-depth consultation with Canadians, drawing on an all-party committee to study the forms of governance and of elections that will serve Canadians' interest not just in the short term, but in many elections to come," Mr. Trudeau said after a caucus meeting on Wednesday. "That's a conversation we feel it's high time to have because our current system is not valuing the vote and input of far too many Canadians."

The NDP also has no plans to submit its proposal for proportional representation to a vote if it wins the next election.

"We'd be looking at consulting with Canadians," Mr. Julian said. "How we consult with Canadians, how we consult the provinces, how we put that into place is something that will be part of the discussions once we get the mandate."