The Conservative Party is vowing to use any means necessary, including a Senate blockade, to keep the Liberal government from forcing through electoral-reform legislation without first holding a referendum.

"The entire Conservative caucus, both in the House and the Senate, will be opposing any radical changes to the electoral system without a referendum" Don Plett, the Conservative Whip in the Senate, said in an interview Wednesday.

"We would look at all avenues" to stop such a bill, interim Conservative Leader Rona Ambrose said. "My hope is that the Liberals will come to their senses."

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The Conservatives are up in arms over a recent declaration by Liberal House Leader Dominic LeBlanc that electoral reform, which would replace the existing first-past-the-post system of electing MPs with some form of proportional representation or a ranked ballot, will simply be passed as a law by Parliament.

The Liberals won a majority in the House of Commons in the past election, but the Conservatives currently have a majority in the Senate. The question is whether the Tories, who may no longer be in that position by the time the bill reaches them, will be able to persuade other senators to join their cause.

However, the non-partisan approach that the Liberals are taking to choosing senators could lead to some new arrivals joining with the Conservatives to block the bill, along with at least a few former Liberal senators whom Mr. Trudeau expelled from caucus.

"I sincerely hope that these independent Liberal senators are also going to rise to the occasion," Mr. Plett said.

Before and during the election campaign, Mr. Trudeau declared that his party was "committed to ensuring that 2015 will be the last federal election conducted under the first-past-the-post voting system."

Under that system, which has been in use since Confederation, the candidate in each riding who gets the most votes wins the seat at election time. Parties that fail to win a majority of the popular vote nonetheless frequently form majority governments, while minority parties are seriously underrepresented in the House of Commons.

With proportional representation, a party's seat total in the House equals its share of the popular vote. In a ranked-ballot system, voters list their preferences from first to last, with the second-choice votes of the least popular candidates reallocated until one candidate has 50-per-cent support.

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The Conservatives insist that the most fundamental change to electing MPs since the introduction of the secret ballot must first be approved by a majority of Canadians.

"This is not about their mandate to govern," Ms. Ambrose said of the Liberal government. "This is about abusing their mandate to govern. I think people will be offended by that. I'm offended by that."

The Prime Minister has said that a special parliamentary committee will consult Canadians before recommending an alternative to the status quo, with legislation to implement those changes promised by no later than May, 2017.

The Conservatives have many questions about the proposed legislative committee. Will it be dominated by Liberals or have equal representation from all three parties recognized in the House? Will the Green and Bloc Québécois parties be invited to participate? How will the committee meaningfully engage with the public on the issue? Will maintaining the status quo be an option?

"I have no preconceived idea of what the electoral system should be," Ms. Ambrose said. "I think there are merits to many of the different electoral systems, including the status quo." But any proposed change must be subject to a referendum, she insisted, even if it has the unanimous support of the committee.

"This is a major change," she said. "This is about what each of our votes means and how it translates into how we are governed."

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There is bound to be a legal challenge to the bill's constitutionality. On the political front, the Conservatives could block electoral-reform legislation through endless debate in the House, known as a filibuster, forcing the government to impose closure.

Once the bill passed the House, it could be vetoed by the Senate, as Mr. Plett has vowed. Liberal senators in 1988 blocked the free-trade agreement between Canada and the United States from passage, forcing Conservative Prime Minister Brian Mulroney to call an election on the issue.

"Our electoral system has served us well for 150 years," Conservative Senator Leo Housakos said in an interview. "It isn't broke, so why bother making it such a big priority to fix it other than the fact that most of us are starting to believe that Mr. Trudeau thinks that by changing the electoral system it would keep them in power in perpetuity."

The current electoral system is thought to favour the Conservatives, while any reform would favour the Liberals and NDP, since those two parties combined routinely win a majority of the popular vote. Electoral reform is, however, so fundamental that it could lead to the creation of new political parties and the disintegration of existing ones.

In any event, a process that the Liberals hoped would be open, consultative and consensual has already degenerated into a fierce fight between conservative and progressive partisans.

Two conservative activists have launched a website and Twitter handle called Defend Democracy, which encourages Canadians to sign a petition demanding a referendum on electoral reform. Referendums on proposed electoral reforms in British Columbia, Ontario and Prince Edward Island all ended in defeat for the Yes side.

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Meanwhile, progressive advocacy groups such as LeadNow and Fair Vote Canada are recruiting supporters and launching their own petitions to fight for electoral reform.