OTTAWA -- Justin Trudeau gave Canada's controversial Trans Mountain pipeline expansion a second lease on life Tuesday, framing the decision in starkly political terms that portrayed his Liberal government as best positioned to walk the narrow tightrope between economic development and environmental protection.

Trapped between the need to find new markets for Canadian oil and his own party's branding as environmental stewards, the prime minister made his best case for a pipeline project -- one the federal government now owns outright -- that critics see as diametrically opposed to the core Liberal message of confronting the climate crisis.

"The truth is, it doesn't make economic or environmental sense to sell any resource at a discount," Trudeau told a news conference in Ottawa -- a reference to the fact that Canadian energy doesn't command a premium on the world market, since the neighbouring U.S. is by far its biggest customer.

"Instead, we should take advantage of what we have, and invest the profits in what comes next -- building the clean energy future that is already at our doorstep. Fundamentally, this isn't a choice between producing more conventional energy or less. It's a choice about where we can sell it and how we get it there safely."

The decision to approve the project a second time comes nine months after the Federal Court of Appeal ripped up the original approval, citing incomplete Indigenous consultations and a faulty environmental review.

The court said the government needed to do better, Trudeau noted. "And you know what?" he said. "They were right."

Critics, of which there are many, wasted little time denouncing the news and predicting another courtroom rejection.

"The federal decision to buy the pipeline and become the owner makes it impossible to make an unbiased decision," said Chief Leah George-Wilson of the Tsleil-Waututh First Nation in B.C. "We will be appealing the decision to the Federal Court of Appeal."

The Liberals ordered the National Energy Board to look at marine shipping impacts; Natural Resources Minister Amarjeet Sohi started another round of consultations with Indigenous communities affected by the project.

Tuesday's decision also comes the day after the Liberals passed a motion in the House of Commons declaring climate change a national emergency that would require more cuts to emissions than have already been promised.

In 2016, the National Energy Board said the production of another 590,000 barrels of oil, which would maximize the twinned pipeline's capacity, could generate 14-17 million more tonnes of greenhouse gases each year, which means Canada would have to find ways to cut more from other sectors to meet and then exceed its current targets.

While he is sympathetic to concerns about the environment and the need to transition to cleaner sources of energy, Trudeau said Canada needs to take advantage of its natural resources while they are still needed in order to fund that transition.

"The policies of the last century will not serve Canadians in this one," he said, acknowledging the concerns of environmentalists who fear the twinning project will exponentially increase the risk of a catastrophic spill on the West Coast.

"I understand your desire to protect your coastline and your ocean, because I share it," he said directly to British Columbia residents. "Our top priority is making sure there is no spill in the first place, but we know we need to be prepared for anything."

Before he was done, however, Trudeau struck a decidedly partisan note, aiming to frame this fall's federal election campaign in terms of which party can best shepherd economic and resource development through the ever-narrowing gauntlet of environmental grassroots opposition.

"The Conservatives built exactly zero pipelines to new markets, but they did manage to build one thing: extreme animosity between those who want pipelines and those who don't."

Trans Mountain, coupled with two Liberal energy bills currently making their way through Parliament, have become the biggest flashpoint between the Liberals and their opponents, with Conservatives demanding the government do more to get the pipeline built and the NDP and Green party urging a full stop.

Green party Leader Elizabeth May and NDP Leader Jagmeet Singh, whose parties are staking claim to the same political territory, were unequivocal in their opposition to Trudeau's decision.

"This announcement represents cynicism and hypocrisy at a level that is quite breathtaking," May said in a statement, citing the Liberal motion passed Monday in the Commons declaring climate change a national emergency.

At the other end of the political spectrum, Conservative Leader Andrew Scheer scoffed at the idea the pipeline would ever be built, and cast doubts on Trudeau's sincerity about supporting the energy industry. "He hasn't done anything," said Scheer. "Show me the pipeline. Where is it?"

The project has also caused major friction between British Columbia and Alberta. Trudeau called both B.C. Premier John Horgan and Alberta Premier Jason Kenney to inform them of the decision Tuesday. Kenney lauded the cabinet decision, but echoed Scheer's skepticism.

"This second approval of the Trans Mountain pipeline isn't a victory to celebrate. It's just another step in a process that has frankly taken too long ," Kenney said.

He urged the federal cabinet to take action on other fronts to get more oil to global energy market, such as accepting proposed amendments on Bill C-69 and abandoning Bill C-48. C-69 overhauls the assessment process for approving major projects like pipelines; C-48 bans oil tankers off B.C.'s northern coast.

The government will require that every dollar in federal revenue coming from the project be reinvested in clean energy and green technology. That includes an estimated $500 million a year in new annual corporate tax revenues once the pipeline is in service, as well as any revenues from the promised sale of the entire expanded pipeline back to the private sector.

The federal cabinet is also requiring another consultation with Indigenous communities affected by the project to determine how they can potentially become economic partners in the project.

The government has accepted the 156 conditions the National Energy Board put on the project approval, but is amending six of them to include requiring Indigenous communities to be involved in developing marine emergency response plans, mitigating potential impacts on sacred sites, and Indigenous involvement in post-construction environmental impact reporting.

There are also eight new accommodations required to take into account Indigenous concerns, including working with some communities to potentially move the route.