Sparks flew over the issue of electoral co-operation, which vaulted to the top of the debate's agenda following May's announcement that the Greens will not run a candidate in the upcoming byelection in Newfoundland and Labrador.
May urged the NDP to stand down as well, leaving the Liberals — who lost the riding by just 79 votes in 2011 — a clear shot at defeating Peter Penashue, the Conservative cabinet minister who resigned after admitting to accepting illegal donations and over-spending during the last election campaign.
May's move played into the hands of Vancouver MP Joyce Murray, the only Liberal leadership contender to advocate a one-time electoral co-operation pact among progressive parties to ensure defeat of Stephen Harper's Conservatives in the 2015 election.
Indeed, Murray claimed credit for convincing May to consider not running a Green candidate in the byelection.
"I am a voice for people who believe that under the old political ways and our current system their voices do not count," she told an overflow crowd of about 1,000 Liberals who turned out to hear the six contenders spar one last time.
"And I'm pleased that in Labrador, the majority will get a progressive MP next time, thanks to speaking about co-operation."
May confirmed that Murray spoke to her last week and "got the issue on my agenda," although the Greens' campaign committee and federal council ultimately took the decision based on their own assessment of the need to ensure Penashue's defeat.
Liberal front-runner Justin Trudeau welcomed the Greens' decision while pouring scorn on Murray's co-operation proposal.
"I encourage all members of the Green party to vote for the Liberal party, in Labrador and across the country," he said.
"The same way I'm going to be encouraging members of the NDP and members of the Bloc (Quebecois) and everyone else to vote for a strong values-based alternative to Mr. Harper, not a hodge-podge that's about winning at all costs."
Murray shot back that Trudeau would perpetuate old style "divisive, toxic politics" that disgusts Canadians and discourages them from voting.
"I can tell you millions, millions of Canadians want this to change, they want a new politics, they want us working together."
Murray called her proposal an "insurance policy" aimed at ensuring defeat of the Conservatives. But she said it's much more than that, aimed ultimately at reforming the electoral system so that each party's share of the popular vote is more accurately reflected in the distribution of seats in the House of Common.
Trudeau retorted that "winning at all costs" is really the definition of old-school politics.
"That's what really worries me about the idea of co-operation because it's a single-minded focus, not on governing but on winning, on taking away power from people we don't like."
Moreover, Trudeau said, "It leads to (NDP Leader) Thomas Mulcair as prime minister and that's not what I'm interested in."
Former Toronto MP Martha Hall Findlay used her closing statement to echo that sentiment.
She noted that Liberals and Greens tried a limited non-compete pact in the 2008 election — in which each party agreed not to run a candidate against the other party's leader — and "it didn't actually work so well."
"Liberals were furious at not having a Liberal candidate to vote for (in May's then-riding of Central Nova), the Conservative vote went up and the Liberal party is still hurting in Nova Scotia as a result."
Murray later attributed all the attention paid to her co-operation proposal as a sign she's the candidate with momentum.
"That's what happens when you're a front-runner."
With Marc Garneau bowing out of the race last week, Murray and Hall Findlay appeared to be jockeying Saturday over who is the second-place candidate. They got into a verbal dust-up over proposed pipelines to carry bitumen from Alberta's oil sands to ports on British Columbia's coast.
Murray, who opposes the pipelines and wants no oil tankers plying the waters off the B.C. coast, accused Hall Findlay of "choosing the interests of the Calgary oil industry over the interests of British Columbians."
Hall Findlay countered "there's nobody up here who has a monopoly on wanting environmental sustainability."
"I do support the oil sands ... The prosperity that we derive from the oil sands benefits all of Canada," she said, adding that pipelines are necessary to get the bitumen to markets.
While Trudeau, Murray and Hall Findlay sparred and jockeyed for position, the other three long-shot contenders had a tough time getting noticed.
Toronto lawyer Deborah Coyne frankly acknowledged her position.
"I have to be honest right now. I am not going to win the race," she admitted after the debate, although she said she intends to remain until the bitter end, promoting her ideas.
Former cabinet minister Martin Cauchon tried to position himself as the candidate best able to reconnect the party with Quebecers. He expressed a willingness to eventually re-open the Constitution in a bid to finally gain the province's signature on the document.
But it was Trudeau's passionate rejection of that idea that won a rousing cheer from the Montreal crowd. He scoffed at the "same, old" notion that special constitutional recognition of Quebec would somehow convince Quebecers "once and for all" that they're fully appreciated within Canada.
"We've been doing that for 30 years and I think that we have to really now admit it doesn't work," Trudeau said.
The best way to put an end to "old squabbles and quarrels," he argued, is to get Quebecers involved with other Canadians in building a common future that reflects their mutual values.
Retired military officer Karen McCrimmon is also in the race.
The party will choose a new leader on April 14th.
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