Prime Minister Justin Trudeau says his government's support for the expansion of the Trans Mountain pipeline was based on what's in Canada's best interest, and will not change even if the position of a new provincial government does.

"The decision we took on the Trans Mountain pipeline was based on facts [and] evidence, on what is in the best interest of Canadians," he said during a joint press conference with Italian Prime Minister Paolo Gentiloni in Rome on Tuesday.

A deal announced Monday between the NDP and the Green Party has poised British Columbia for a change in government, following confirmation of a tight provincial election result earlier this month that saw Christy Clark's Liberals fail to win the majority of seats in the legislature.

More details on the scope of this agreement are expected Tuesday, but the two parties campaigned against expansion of the Kinder Morgan pipeline. Clark's government was supportive.

"Regardless of a change in government, in British Columbia or anywhere, the facts and evidence do not change," Trudeau said. "We understand that growing a strong economy for the future requires taking leadership on the environment.

Rosemary Barton, host of CBC’s Power & Politics, breaks down the project and the challenges it has encountered to date 1:12

"We have to do those two things together. That is what drives us in the choices we make, and we stand by those choices," the prime minister said.

Provinces 'cannot unilaterally stop projects'

In a statement early Tuesday, Alberta Premier Rachel Notley said she's confident the two provinces could work together on important issues should the new accord result in an NDP-led government in B.C.

But "it is no secret that we have one important disagreement," she said, describing the Trans Mountain expansion as "critical" to both Alberta's economy and the national economy.

Alberta Premier Rachel Notley, seen here at an energy infrastructure announcement near Carstairs, Alta., earlier this month, continues to push for the Trans Mountain expansion. (Jeff McIntosh/Canadian Press)

"It comes with significant safety measures that will better protect Canada's West Coast and Alberta's commitment to a world-leading climate plan," the statement said. "Because of that, the National Energy Board and the federal government, which has ultimate responsibility, approved it after a rigorous environmental review."

Notley said provinces "do not have the right to unilaterally stop projects such as Trans Mountain that have earned the federal government's approval. This is a foundational principle that binds our country together.

"There are no legal tools available to provinces to stand in the way of infrastructure projects that benefit all Canadians. We will use the means at our disposal to ensure that the project is built."

Federal MP Nathan Cullen, who represents a coastal riding in northern B.C,, said he was confident that his fellow New Democrats Notley and John Horgan, who could be B.C.'s next premier, would "find some ways to have a conversation about this."

In the meantime, he reminded reporters Tuesday that the federal Liberals had also broken their promise for a credible environmental review of the expansion.

"Clearly it's proven to be a major thorn in the side of British Columbians who are being asked to take an extraordinary amount of risk for very little to no benefit," Cullen said.

"It was never a sure thing," he said. "This project has always had problems. Pipelines don't go to IPOs in my experience and then have to downgrade their offering before they go to tender."

12 legal challenges underway

Also in Ottawa, Green Party Leader Elizabeth May said the B.C. government has the right to do its own environmental review of the project. Provincial permits are required for river crossings along the pipeline expansion route.

"I don't believe a full-blown constitutional battle between British Columbia and Canada will be in anyone's interests," she said, saying she didn't want to pre-judge what may be in the agreement between the NDP and the B.C. Green party, making it clear that she doesn't speak for her provincial counterpart.

"I don't think that Justin Trudeau wants to expropriate a pipeline route," she said. "I don't think they want to be declaring that kind of top-down, federal-provincial expropriation. I can't see that happening. It's too extreme."

Green Party Leader Elizabeth May took part in the Walk for the Salish Sea in Victoria last week. The protest started at the pipeline's Mile 0 and made its way to the Kinder Morgan Westridge terminal in Burnaby. (Chad Hipolito/Canadian Press)

May pointed out there are many challenges before the courts that could see the federal permits for the pipeline's construction quashed for violations of procedural fairness during the National Energy Board's hearings.

Indigenous rights issues are also being challenged in court.

A new provincial government may intervene in these cases to support the First Nations, municipalities and environmental groups who want the expansion stopped.

"There are about 12 different significant legal challenges already before the courts on different grounds. Any one of which could quash the permits," she said. "I think it's dead."

Private landowners may want compensation if the pipeline goes ahead.

"I wouldn't want to invest in it. And I sure don't think that they can go forward to investors on their IPO offering and say they have all the permits in yet," she said. "This is a long way from a done deal."

'They just want him to stay away'

"We've been clear from the start that this pipeline has federal approval," said Natural Resources Minister Jim Carr after cabinet Tuesday. "We will be patient and we will wait for the process in British Columbia to work itself out."

Later in question period, Carr said the federal review of the Trans Mountain expansion was "the most exhaustive in the history of pipelines in Canada." The NEB made 157 recommendations, while a ministerial panel went up and down the line consulting with people.

"The consensus, after all of that conversation with Canadians, was that it is in the national interest to build that pipeline," Carr said.

BC pipelines top Question Period Tuesday 2:34

"I think the prime minister's been put in a very difficult position," said B.C. Conservative MP Mark Strahl. "He said on Nov. 30 when he approved this pipeline that he would not let any political arguments stand against it.

"He now has to champion the project or it won't get built," he said. But federal Liberal MPs from B.C. don't want Trudeau campaigning.

"They just want him to stay away. I think they'd be quite happy if this pipeline was killed," Strahl said.

At least one B.C. Liberal MP disagreed his caucus was divided.

"On the ground, people want it. People want economic prosperity in British Columbia and that is what we are looking forward to," said Sukh Dhaliwal.