The NDP would categorically refuse to prop up a Conservative minority in the House of Commons, making it even harder for Stephen Harper to stay on as Prime Minister if he does not win a majority on Oct. 19.
NDP Leader Thomas Mulcair said there is not "a snowball's chance in hell" that he would help the Conservatives to govern in a minority environment. The comment comes one day after Liberal Leader Justin Trudeau said he would not support a Conservative minority in the House.
The Bloc Québécois has also stated at the start of the election that it would not prop up the Conservatives.
As a result, the Conservatives would likely need to win a straight majority of the 338 seats up for grabs in this election to form a government.
"Anybody who has attended a single Question Period over the course of the last several years would be able to tell you that there is no likelihood that the NDP would ever, under any circumstance, be able to support Mr. Harper," Mr. Mulcair said on Wednesday, at a joint news conference in Montreal with Mayor Denis Coderre. "The short answer to your question is: There isn't a snowball's chance in hell."
In reference to an eventual collaboration with the Liberals, Mr. Mulcair was less categorical, stating that "every time we have tried, they have slammed the door."
Speaking in Montreal on Tuesday, Mr. Trudeau said if he doesn't win the next election, the Liberals would not prop up a Conservative minority government.
"There are no circumstances in which I would support Stephen Harper to continue being Prime Minister of this country," Mr. Trudeau said.
He was less emphatic in reference to the NDP, stating Parliament can function in the context of a minority government and he is confident that Canadians will make the "right choice" on Oct. 19.
Mr. Harper has said that he would not try to form a government if the Conservatives were to finish in second place, as would be his constitutional prerogative. It remains to be seen what he would do in the event of a minority.
For now, Mr. Harper has urged voters to offer his party a "solid, stable, national majority" in the election.