In a rare show of sparks between the leadership candidates, the British Columbia politician found himself on the defensive Sunday after Thomas Mulcair, Peggy Nash and Martin Singh slammed the idea.
"Why don't you think it's enough to get progressives to rally around our banner?" Mulcair asked Cullen during the debate in Halifax.
"Why would you shoot for the bronze medal when you know that Canadians share our goals, share our vision?"
Nash said the proposal would strip Canadians of their democratic right to vote for the party of their choice.
"We had a historic breakthrough in the last election. Why not build on that?" the Toronto MP said, generating a round of applause from the audience at Citadel High School.
"Let's not look at a rear-view mirror."
In response, Cullen said his ultimate goal is to oust the federal Conservatives from power and bring in a proportional representation voting system if he were elected prime minister.
"Let's all recognize the thing that we know: that the current voting system we have in this country is broken and flawed," he said.
"No one can question my faith in this party. No one can question my work ethic over the last number of years in building up in places that we thought we had no hope."
Cullen has said he would allow local NDP riding associations to decide whether it's in their interest to hold joint nomination meetings with other parties in Tory-held ridings.
After the debate, Cullen said he would not back down.
"In fact, we're doubling down on it," he told reporters, though he added he is not "married to the details."
"The particulars of how this thing happens — it can happen in many different ways."
In another heated moment at the debate, Ottawa MP Paul Dewar asked Mulcair to clarify his position on the sale of bulk water. Mulcair said he is in favour of protecting water resources and opposed to exporting it.
"But you know ... that's my position," said Mulcair, a former minister of environment in Quebec.
Niki Ashton, Brian Topp and Romeo Saganash are also vying to replace the late Jack Layton.
Each of the contenders took turns attacking the federal Conservative record on health and pensions in what could be a prelude for what to expect when the House of Commons returns Monday.
Topp, a former NDP president, called Prime Minister Stephen Harper a hypocrite for saying he supported families while "kicking them in the shins" with proposed changes to Old Age Security.
The Tories have said they want to ensure OAS remains sustainable, but haven't confirmed speculation they will raise the eligibility age to 67 from 65.
Other debates every two weeks are planned in Quebec City, Winnipeg, Montreal and Vancouver. NDP members will choose their new leader on March 24 at a convention in Toronto.
Layton died of cancer last August just months after leading the NDP to 103 seats and official Opposition status for the first time in the party's history.
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