OTTAWA - Liberal leadership hopeful Joyce Murray says electoral co-operation with the Greens could be a "game-changer" that ensures the defeat of the Harper Conservatives -- even if New Democrats refuse to go along.

NDP Leader Tom Mulcair has flatly rejected any kind of collaboration with the Liberals, whom he deems untrustworthy.

But Murray thinks Mulcair could still change his mind, persuaded by the many New Democrats -- including House leader Nathan Cullen -- who support co-operation.

Even if he doesn't, the Vancouver MP says that wouldn't cripple her proposal for a one-time non-compete pact among progressive parties in vulnerable Conservative ridings during the 2015 election.

The Liberals could still join forces with the Greens, whose leader, Elizabeth May, is an enthusiastic advocate of the idea.

Had Liberals and Greens united behind a single candidate in last November's byelection in Calgary Centre, Murray says they could have prevented Tory Joan Crockatt from eking out a narrow victory with just under 37 per cent of the vote.