OTTAWA - NDP leadership contender Nathan Cullen wants New Democrats to join forces with Liberals and Greens in some ridings to defeat Stephen Harper's Conservatives.

The British Columbia MP proposed Tuesday that his party enter into non-compete deals with other "progressive" parties in some Tory-held ridings.

Cullen said he doesn't support a full-blown merger with the struggling Liberals.

But he said co-operation is necessary to ensure defeat of Prime Minister Stephen Harper's Conservatives.

"I have no problem going up against Stephen Harper one on one," Cullen told a news conference.

"This just makes it a slam dunk."

Under Cullen's proposal, the NDP would identify ridings where the Tories could be beaten if so-called progressive parties united behind a single candidate.

New Democrats in those ridings would be asked to decide whether they want to run a joint candidate.

Cullen made it clear he wouldn't expect the NDP to unilaterally disarm; his scheme would only work if other parties agreed to participate in joint nominations of candidates.

He challenged the NDP and other parties to put down their "bayonets for a moment in order to get something larger than ourselves accomplished."

"I believe now is the time to respond to the call from people of all walks of life, who hate that Stephen Harper can change our country for the worse with support of less than four in 10 voters," Cullen said.

"For me, the greater cause is that the wedge politics of the Stephen Harper government are killing us. ... We need to find a way to speak past the narrow political interests of parties from time to time."

Cullen said he's broached the idea of joint candidates with grassroots New Democrats and found "there's hesitation at first and then a great surge of enthusiasm."

There was little evidence of that Tuesday from his fellow leadership candidates, however. Nor was there much discernable enthusiasm from Liberals.

Only Green Leader Elizabeth May seemed immediately receptive.

"It is certainly a step in the right direction to strike a more co-operative tone," she said in an email.

While her party's members would need a "deep discussion" about the idea, May added: "The risks of a continued Conservative majority (to the environment, to science, to Canada's role as a peacekeeper) are too significant to ignore any olive branch."

Interim Liberal Leader Bob Rae declined to comment on what he termed "an internal NDP issue." But most Liberals are so dead set against any deals with the NDP, party brass compelled Rae to agree not to discuss merger or co-operative arrangements as a condition of becoming interim leader.

Three of Cullen's rival leadership contenders also turned thumbs down on Cullen's proposal.

"I'm impressed that Nathan Cullen is looking for innovative ideas, but I don't support this one," said former party president Brian Topp.

Topp said the party "owes New Democrat voters and all Canadians a clear, coherent platform and a unified team of candidates — who know what team they're playing on — in every riding in the country." And he questioned how joint candidacies would work.

"On what platform would joint candidates run? Who would they support to govern the country?"

Montreal MP Thomas Mulcair said Cullen "often has interesting ideas, even when I don't agree with them, which is the case here."

Ottawa MP Paul Dewar said the NDP should be focused on growing its support so that it can defeat the Tories in the next election. He suggested Cullen's proposal is counter-productive, encouraging people to think they don't necessarily have to join the NDP to stop Harper.

"I think we need to run candidates in all ridings," Dewar said.

Cullen did get some support from Winnipeg MP Pat Martin, who has threatened to run for the leadership himself if other candidates don't embrace the idea of working more co-operatively with the Liberals, including potentially merging the two parties.

Martin called Cullen's proposal "fantastically refreshing" but said he still hasn't ruled out a leadership bid of his own.