Green Party Leader Elizabeth May says she wouldn't prop up a minority government that moves forward with the Trans Mountain pipeline expansion.

"If the positions of the other parties were cast in stone and they would never budge, none of them would be supported by Greens," May told CBC News on Wednesday aboard a Via Rail train rolling through Nova Scotia while campaigning for next month's federal election.

"It is not about politics," May added. "It's about a very clear imperative that global average temperature increase going above 1.5 degrees Celsius is so dangerous that the survival of a healthy biosphere is in the balance."

Asked whether there was any way her party might change its position on the pipeline in a minority situation, May was emphatic: "The answer is no."

May said she's confident the Greens will pick up several seats in the Maritimes. Those seats, she said, could determine which party forms government if there's no clear majority after Oct. 21.

Minority governments led by either the Conservative or Liberal parties remain the most likely outcomes at this point of the campaign, according to analysis by Eric Grenier for the CBC Poll Tracker. In a minority Parliament, the support of the Greens or the NDP — through an informal arrangement or coalition, or on an issue-by-issue basis — could determine which party is in a position to form a government with the confidence of the House of Commons.

Coalitions have been exceedingly rare in Canadian politics, although the current minority NDP government in B.C. governs in an informal alliance with the Greens.

NDP Leader Jagmeet Singh said in August he would not support a Conservative minority.

Speaking in Campbell River B.C. Thursday Singh was asked if he would support any government that was determined to proceed with the Trans Mountain pipeline.

"I'm opposed to the project, absolutely and fully opposed to it, so whatever government the people of Canada choose, I will be doing my best to continue to fight this project," Singh said. "I will work with whatever position I'm in to continue to oppose this project."

Elizabeth May greets supporters at a rally at a rail station in Amherst, N.S. on Sept 25, 2019. (David Thurton/ CBC)

Both the Conservatives and the Liberals are committed to building the Trans Mountain expansion. The Liberal government purchased the pipeline for $4.5 billion in 2018 after the project's proponent, Kinder Morgan, announced it would not go ahead in the face of opposition from the B.C. government, some Indigenous communities and environmental activists.

The expansion, which would carry diluted oilsands bitumen from Alberta to British Columbia's coast, would boost the pipeline's existing capacity from 300,000 barrels a day to 890,000 barrels. Once the pipeline is twinned, it would generate 400,000 tonnes of greenhouse gases annually — an amount the Liberals say wouldn't exceed Canada's greenhouse gas reduction targets.

May has said in the lead-up to the fall election, and throughout the federal campaign, that she wouldn't form a coalition with the Liberals or the Conservatives unless they have serious plans to tackle climate change. But this is the first time the Green Party leader has said that support from her party would be contingent on abandoning Trans Mountain.

May says parties might reconsider their position on pipelines

May suggested during the interview that parties committed to expanding the pipeline might have a change of heart — or even leadership — if faced with the prospect of going back to the polls because they can't form a government.

Speaking in Montreal Thursday, Green Leader Elizabeth May says her party is prepared to defeat a government that priorities industry over the environment. 0:47

"We know that positions change," May said. "And we even know that sometimes a leader of a federal party through a federal election campaign looks at disappointing results and steps aside at the night of the campaign."

May was asked if she would step down if the Greens do badly on Oct. 21, "That's a trivial question," she replied. She said she would resign if the party's routine leadership review after the election showed she didn't have the support of most members, but she would remain an MP.

"I am always interested in looking for the strongest possible leadership," May said. "I would be more than happy to pass that on and work with a new leader in a larger caucus of Greens for a full term and another one after that."