If there's anything unifying the opposition leaders, it's this — none of them wants to play government with Stephen Harper.

Should the Conservative leader win a minority of the parliamentary seats on Oct. 19, all have vowed to try to oust him at the first opportunity, whether through a non-confidence motion, or, as NDP Leader Tom Mulcair has suggested, a coalition. 

Not surprisingly, this campaign, like most, has been marked by hostility, much of it, targeting the prime minister. Yet there's been a fair amount of animosity between Mulcair and Liberal Leader Justin Trudeau. And it raises the question of whether they would be able to put those feelings aside to topple Harper.

With only days left in the campaign, the topic of a minority government and the political machinations that would follow inevitably comes up in one form or another.

Bloc Québécois Leader Gilles Duceppe was asked about it Tuesday, prompting this reply: "Stephen Harper will not be prime minister, even if he finishes with the most seats, in a minority Parliament."

At a campaign stop Monday, when Trudeau was asked about it again, said he got into politics because he deeply disagreed with Harper's vision of Canada, and that there were no circumstances in which he could support his government or stand back and allow him to be prime minister.

'Snowball's chance in hell'

Mulcair, who said earlier in the campaign there's not a "snowball's chance in hell" he'd support a Harper minority government, said Tuesday that "I want him gone." During a Q&A session with Vice Canada, he said he would "do whatever I have to do" to make sure Harper never serves another day as prime minister after Oct. 19.

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NDP Leader Tom Mulcair says he would 'do whatever I have to do' to make sure Stephen Harper never serves another day as prime minister after Oct. 19. (Ryan Remiorz/Canadian Press)

But that means there would have to be some kind of agreement or working arrangement between Mulcair and Trudeau. And during the campaign, the two rivals have taken numerous shots at each other.

Mulcair has criticized Trudeau's support for the anti-terrorism Bill C-51 and the Keystone XL pipeline and his plan to run three consecutive deficits. 

Trudeau has accused Mulcair of being a political opportunist, of saying one thing to a French audience and another to an English crowd and of flip-flopping on a series of issues.

That two political competitors would criticize one another is hardly shocking. But the political friction between the two seems to run a little deeper.

Some of that surfaced at the debates in mocking jabs. When the Liberal leader suggested Mulcair's promises were "puffs of smoke" Mulcair responded: "You know a lot about that, don't you, Justin," a shot at Trudeau's support for legalizing marijuana. 

When Trudeau said Mulcair had announced his climate change plan with "pomp," Mulcair snapped back, "I'll leave the pomp to you, Justin."

And when Mulcair compared Bill C-51 to Pierre Trudeau's invocation of the War Measures Act during the 1970 October Crisis, Trudeau replied that he was "incredibly proud to be Pierre Elliott Trudeau's son." He went on to list some of his father's political achievements, including bilingualism, which "as my father understood Mr. Mulcair, means saying the same thing in French as you say in English."

Asked Tuesday by a reporter whether he has refused to discuss with the NDP leader co-operation in a minority government, Trudeau instead turned the question around to criticize some of Mulcair's policy choices.

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Liberal Leader Justin Trudeau says there are no circumstances in which he could support Stephen Harper's government (Paul Chiasson/Canadian Press)

Trudeau has repeatedly said he is not interested in any formal coalition with the NDP — stressing that he is "unequivocally opposed."

He has said, however, when it seemed more likely that the NDP would run second, that he might support an NDP government on a case by case, or bill by bill, basis. 

Bad blood

On Tuesday, Mulcair was asked whether there was just too much bad blood or too many policy differences with the Liberals that would preclude him from working with them.

Mulcair pointed out that in 2008 he had helped then NDP leader Jack Layton write a formal coalition agreement with the Liberals to get rid of Harper. And it was the Liberals, Mulcair reminded, who walked away from that deal.

"I knew what my priority was in 2008 when that crisis hit, was to get rid of Stephen Harper," Mulcair said. "I know what my priority is in this election. It's to get rid of Stephen Harper. Every time I've opened that door to the Liberals, it's Justin Trudeau who has taken it on himself to slam that door shut."

Mulcair said Trudeau is making it personal when he says he can work with the NDP but not with him.

"They have been letting me have it since the beginning of the campaign, that's a big difference," Mulcair said. "Justin Trudeau's been fighting me more than he has been fighting Stephen Harper. My opponent is Stephen Harper in this campaign."

It will certainly be challenging for the two to work together. For example, the Liberals immediately put out a news release after Mulcair's comments, saying that he "falsely claims he does not criticize Liberals," that there are "30 negative references" to Liberals or Trudeau in the "flawed NDP platform" and that it just demonstrates that he will "say whatever is politically convenient."

But despite all the political rhetoric and hostility, neither really has much of a choice — if they want to topple a minority Harper government, they will be forced to find a way through it.

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With files from The Canadian Press