The possibility of co-operation – and perhaps even a merger – with the New Democrats as a means to defeat Conservatives in the next federal election is promising to become a key battle line in the looming Liberal leadership race, whether the Liberals like it or not.

At least one potential contender is already saying he would campaign for co-operation. David Merner, a Liberal for more than 30 years who recently quit as president of the British Columbia wing of the party to test the leadership waters, says the issue would be at the heart of his campaign.

Mr. Merner, an admitted dark-horse candidate, said he supports the ideas floated by New Democrat MP Nathan Cullen during the NDP leadership race that federalist parties on the centre and left should nominate candidates jointly at the riding level. "We are no longer the governing party," Mr. Merner said. "And we've got to look at how we co-operate."

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No less than former prime minister Jean Chrétien has publicly promoted the idea of Liberals and New Democrats merging, saying it would bring stability to federal politics. Others are not so public. Another potential candidate, who would not be named, told The Globe and Mail he would recommend discussions to ensure that vote-splitting between the two parties does not hand the Conservatives another victory in 2015.

The idea is not welcomed by many long-time Liberals, including a number who have expressed an interest in the leadership. They rule out any form of co-operation, saying they they want their party to regain prominence.

But even if most paid-up members of the Liberal Party of Canada oppose a union in which their banner is swallowed by the stronger New Democrats, nothing prevents someone from entering the leadership race simply to bring about a meld. Because the Liberals will allow non-members to vote, the final choice of leader will not rest only in the hands of people who have a strong devotion to the party's continued existence.

When asked if a candidate could run on a pro-merger ticket and win with the help of non-Liberal voters, party spokeswoman Sarah Bain said: "It will be a wide-open race and we leave the speculation to the speculators."

Pollsters like Nick Nanos of Nanos Research point out that if the two parties join together, right-leaning Liberal voters might slip away to the Conservatives, ensuring another Tory majority. Still, a national poll conducted last month suggested that more than half of federal Liberal and NDP supporters backed the idea.

To this point, NDP Leader Thomas Mulcair says co-operation is not an option. But his party is riding high in the polls. His thoughts could change if NDP support takes a nosedive. And, according to party insiders, Liberal and NDP riding executives are already starting to work together in parts of Ontario and Alberta.

Organizations on the centre and left of the political spectrum say they will urge candidates to talk about breaking down barriers between the Liberals and the NDP.

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"We are, as you know, non-partisan but deeply concerned that a Canada that stands for social justice, human rights and environmental sustainability cannot survive another Harper majority," said Maude Barlow, national chairperson of the Council of Canadians. "The NDP has a strong, progressive leader. If the Liberals decide to run someone to challenge him, Stephen Harper could very well get another term. The progressive elements of our society have to come together in solidarity."

And Jamie Biggar, the executive director of Leadnow.ca, which has challenged measures of the Harper government, says his group advocated for co-operation with the Liberals during the NDP leadership campaign and will do so again as the Liberals pick a leader.

"We believe there is broad support for this idea amongst Canadians, particularly those who are interested in politics but turned off by hyper-partisanship, and we are constantly hearing from people who want to organize to support this idea in their ridings," he said. "Close to 10,000 people joined the NDP to support co-operation for electoral reform, and we can keep building major support in the three years leading up to the 2015 election."