Elizabeth May says the Greens have always urged co-operation between parties on the centre and the left, and Etobicoke Centre would be a good place to start if a Supreme Court ruling makes a new vote in that Toronto riding necessary.

The Green party Leader said in a telephone interview this week that she would urge her party not to run a candidate in the contest and she thinks the New Democrats should also skip any repeat of the race which was won in 2011 by Ted Opitz, a Conservative, who beat Liberal candidate Borys Wrzesnewskyj by 26 votes. The Greens and the New Democrats were far behind.

Although Ms. May she said would not normally urge her party to stay off a ballot, the situation in Etobicoke Centre is highly unusual. If anyone was unfairly denied a seat in that riding it was Mr. Wrzesnewskyj, she said, and if there is a by-election it should be "a clean vote between Borys and Ted."

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The Supreme Court is expected to rule shortly on an appeal by Mr. Opitz of a lower-court decision to toss out the results in Etobicoke Centre after the judge concluded that 79 voters cast ballots without evidence that they were properly cleared to do so. The case is being deliberated as some members of all parties on the left and centre discuss whether there is any room for electoral co-operation in advance of the 2015 election as a way of preventing a fourth Conservative victory.

Ms. May, who stressed that she is more interested in dealing with the legislation that the Conservatives will introduce in the fall than planning for an election that is more than three years away, said Etobicoke Centre and by-elections that will take place in Calgary Centre and the Ontario riding of Durham will be interesting tests of the parties' willingness to work together.

Other than Etobicoke Centre, which she would sit out altogether, Ms. May said she could envision situations in which all parties run candidates but agree that one of them has the edge and direct all of their efforts to getting that person elected.

The key, she said, is whether NDP Leader Thomas Mulcair is willing to talk about it. Mr. Mulcair , whose party is currently riding high in the polls, has ruled out any form of co-operation and a party spokesman said Tuesday that the New Democrats did intend to campaign hard in Etobicoke Centre.

But Nathan Cullen became a frontrunner in the NDP leadership race by urging joint candidate nomination meetings with the Liberals and the Greens. And there is already co-operation taking place at the riding level, particularly in Calgary Centre, said Ms. May.

"Lots of local grassroots Liberals and NDPers and Greens are all saying 'What can we do?'" she said. "I think at the national level, it would be great to sit down and have a conversation with Tom Mulcair and Bob Rae and say, 'We've got three parties in opposition that can play roles in these by-elections.'"

The Green Party Leader said her party's federal council, and possibly even the local candidates, would have to approve any co-operative efforts with other parties. But "I see us as the party that tries to promote co-operation," she said. "We work well with the NDP, we work well with the Liberals. Trying to draw those two parties together to have a conversation is something I think we can do."