Liberal leadership hopefuls sparred today over an existential issue for their once mighty party: can it win the next election on its own or does it need to join forces with other progressive parties?

The issue of electoral co-operation produced the only real sparks during the first of five leadership debates in Vancouver.

It was a relatively civil affair, with no real punches thrown but a few veiled jabs at front-runner Justin Trudeau over his inexperience and alleged lack of intellectual gravitas.

However, the number of candidates -- nine -- and a format that dwelled repetitively on only a few issues gave Trudeau's lesser-known rivals little opportunity to really make their mark or narrow the Montreal M-P's perceived lead.

All nine ruled out a merger with the N-D-P. Indeed, eight of the nine insisted the Liberals, reduced to a third-party rump in the last election, need no outside help at all to rebound in 2015.

Vancouver M-P Joyce Murray alone embraced electoral co-operation, touting her plan to let Liberals join forces with New Democrats and Greens in ridings where a united front could defeat the ruling Conservatives in the next election.

She said it would be a one-time initiative aimed at replacing the Tories with a progressive government that would implement electoral reform. It would end the current first-past-the-post system that results in parties winning majorities with less than 40 per cent of the vote.