VANCOUVER - The ever-simmering world of pipeline politics is once again threatening to boil over after the federal government approved the Trans Mountain project, as both backers and critics of the polarizing proposal mobilize for what many predict will be a fierce and lengthy battle.

While a litany of local politicians, First Nations and environmental groups have come out in opposition to Trans Mountain, some observers predict Vancouver Mayor Gregor Robertson will emerge as a key figure in the fight to block the $6.8-billion initiative.

The Kinder Morgan project involves expanding an existing pipeline between Edmonton and Burnaby, a city that borders Vancouver. The move would triple the line's capacity and lead to a sevenfold increase in tanker traffic along a heavily populated section of the B.C. coast.

Karen Mahon, national director of Stand.Earth, said Robertson's business background and green credentials add valuable credibility to a cause some dismiss as ideological.

“He understands the importance of the natural world, deeply, and holds it close to his heart,” said Mahon.

“But he's a businessman. A lot of his life work is about how to meet our economic realities while protecting our natural resources and our children's inheritance.”

Robertson was not available for comment.

The future mayor's business career took off in the early 1990s when he co-founded organic juice company Happy Planet. Robertson entered politics in 2005, winning a legislature seat for the NDP before stepping down three years later to run in Vancouver's mayoral race.

Now halfway through his third term as mayor, the 52-year-old has said he expects to seek a fourth mandate in 2018.

Moe Sihota, a former NDP provincial cabinet minister, describes Robertson as a formidable political force.

“His understanding of the business environment in the province, as well as his social (and environmental) conscience, are real attributes that I think make him a strong public figure,” said Sihota.

“Every cause needs a crusader. And he's that.”

But Robertson's stance on Trans Mountain pits him against powerful opponents. He is at odds with Premier Christy Clark, as well as Prime Minister Justin Trudeau, with whom he is reported to have a friendly relationship.

The two have been described on social media and news outlets as sharing a “bromance.” When Robertson turned 50, Trudeau, then Opposition leader, sent the mayor a playful video message wishing him a happy birthday.

Robertson already enjoys a relatively high national profile thanks to various sustainability initiatives he introduced as mayor, including Vancouver's Greenest City 2020 Action Plan, which has garnered international recognition.

Earlier this year, the mayor was invited to meet with Pope Francis for a climate change summit in Vatican City. He was the only Canadian present.

Not everyone lauds the mayor's environmental activism, with some criticizing him for focusing too much on global challenges at the expense of local issues.

“I wish the mayor would spend as much time in the city working on city business as he does protesting and advocating on issues that are unrelated to the city of Vancouver,” said Coun. George Affleck, a two-term civic politician who ran opposite Robertson's slate in the two most recent elections.

“In (Trudeau's) case, he's thinking about the whole country,” Affleck added. “The mayor should be thinking about Vancouver.”

Coun. Geoff Meggs, a political ally, described the mayor as uniquely placed to have an impact on the Trans Mountain dispute. He has a good relationship with Ottawa, Meggs said, and his background as a New Democrat may be an advantage when interacting with Alberta's NDP premier, Rachel Notley.

Notley is an advocate for the pipeline, which would offer a much-needed jolt to her province's moribund economy by opening up access to oil markets across the Pacific Ocean.

Trans Mountain still faces legal and regulatory hurdles beyond federal approval. Several lawsuits have been filed in Federal Court challenging the assessment process, and the project must still obtain construction permits from municipalities through which the pipeline would run.