NDP Leader Tom Mulcair still isn't ruling out forming a coalition with the Liberals following this fall's federal election, despite Justin Trudeau's continuing refusal.

New Democrat MP Nathan Cullen kicked off a renewed debate over the value of a coalition when he said that while winning a majority in this fall's federal election is still his party's goal, ultimately the No. 1 priority is toppling the Tories.

"The Liberal voters that I know are as fed up with Stephen Harper as anybody," Cullen, the MP for British Columbia's Skeena-Bulkley Valley riding, said in an interview on Wednesday.

Trudeau has previously rejected the idea of an NDP-Liberal coalition, though earlier this year he said he might be more open to the possibility if Mulcair were not at the party's helm.

Liberal Leader Justin Trudeau, left, said again Thursday he is not interested in a "formal coalition" with the NDP under leader Tom Mulcair, after B.C. NDP MP Nathan Cullen said the Liberals would have to consider the idea to unseat Prime Minister Stephen Harper. (Ryan Remiorz/Canadian Press)

On Thursday, Trudeau said the parties could only work together on legislative bills.

"There are a number of issues on which the Liberal Party of Canada and the NDP disagree in quite a fundamental level. So although of course we are open to working with all parties in the House to pass good legislation and to ensure that Canadians' interests are served, there will be no formal coalition with the NDP," he said in Winnipeg.

Mulcair reiterated that his party's priority is to defeat and replace the Conservative government.

"We know that they've done a lot of harm and we want to start repairing the damage that they have done," Mulcair said in Amherstburg, Ont.

"We're going to let the Canadian voting public decide and whatever form that Parliament takes is the one that we'll work with."

Trudeau risks 'damage'

The last time the idea of a coalition government was seriously floated was seven years ago, when the NDP, Liberals and Bloc Québécois came together to force the government out of office.

Their efforts were thwarted when the Governor General, at the prime minister's request, prorogued Parliament, effectively putting it on pause until the new year, by which time there had been a change in Liberal leadership.

NDP MP Nathan Cullen says Justin Trudeau risks offending progressive voters if he's unwilling to consider a coalition with New Democrats to stop Stephen Harper's Conservatives from continuing to govern. (Adrian Wyld/Canadian Press)

Newly chosen Liberal Leader Michael Ignatieff ultimately backed out of the merger by grudgingly supporting the Conservative budget, saving Harper from losing a confidence vote and being forced to call an election.

"I think the Liberals lost their nerve last time and made a huge mistake," said Cullen.

"But Justin Trudeau will do himself a great deal of damage with progressive voters if he wants to contemplate more years of this Harper government."

Coalition governments are relatively common in other parliamentary democracies, such as Germany, but they occur far less frequently in Canada, where the first-past-the-post electoral system favours the formation of majority governments.

Cullen said voters' hunger for change would overcome any potential discomfort with the relatively unfamiliar political arrangement.

"Canadians are going to reward those parties that are willing to work with others and work on behalf of the country first," he said. "Our eyes are focused on our opponents, and our opponent is Stephen Harper right now."

Voters are expected to go to the polls on Oct. 19, as per Canada's fixed election date law. However, nothing prevents the prime minister from asking the Governor General to dissolve Parliament and send Canadians to the polls earlier.