Prime Minister Justin Trudeau's approval of the Trans Mountain pipeline expansion between Alberta and B.C. is testing the popularity that helped his Liberals win a record 17 of 42 British Columbia seats in the last federal election.

Now, some Liberal MPs elected and re-elected in B.C. in 2015 are uncertain about how the party will do in 2019, rattled by pointed, vocal anger among voters concerned about possible spills as a consequence of increased tanker traffic carrying crude oil from the pipeline's terminus in Burnaby.

On Wednesday, some MPs said they were disappointed and apprehensive about the decision to support the $6.8-billion project, and now worried about holding onto voter support in one of Canada's most competitive electoral battlegrounds. Massive public protests are expected.

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Liberal MP Ron McKinnon, who recommended against approving Trans Mountain, said the only option is to make the best of the situation.

"The decision is made. It will not be undone. The time for arguing the pros and cons has passed," the Coquitlam-Port Coquitlam MP said in a statement.

Joyce Murray, the MP for Vancouver Quadra, told reporters in Ottawa she was "disappointed" by the federal decision and her riding feels a lot of "angst" over the project.

Ms. Murray said a "clear majority" of her constituents oppose the pipeline expansion, and noted those residents are being outspoken. "It's my staff in my office in Vancouver that are bearing the brunt of people's anger and sense of betrayal and concern," she said.

Hedy Fry, Vancouver Centre's MP for 23 years, said Trans Mountain is among the most significant issues she has seen as an MP, partly because Vancouver Mayor Gregor Robertson and First Nations as well as many of her constituents oppose it.

In an interview, Ms. Fry said she was unhappy and disappointed, but the best move now is for MPs to highlight the $1.5-billion oceans-protection plan the Prime Minister announced during a recent Vancouver visit and make sure it is enacted. "That's not chump change," she said.

Federal Transport Minister Marc Garneau, visiting Vancouver on Wednesday, agreed. "In the months and the years to come, we will show people in British Columbia just how seriously we take the environment off this coast and all of the coasts in Canada."

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Defence Minister Harjit Sajjan, MP for Vancouver South, said Mr. Trudeau has always encouraged MPs to express the concerns of their constituents.

In announcing the decision on Tuesday, Mr. Trudeau mentioned Burnaby-North Seymour MP Terry Beech, who had been public about his opposition to the expanded pipeline. The Prime Minister called Mr. Beech one of the "most articulate and substantive critics" of the proposal. Mr. Beech has said he will have an open house at his constituency office this weekend to hear what people have to say.

Despite signs of unease on Wednesday, Mr. Sajjan said the B.C. caucus is united.

Mr. Sajjan said the prospect of protests will not deter Mr. Trudeau from future visits to British Columbia, where he has largely been greeted by exuberant crowds. "He will be coming to B.C. like he has always done," he said.

But former federal cabinet minister David Anderson said on Wednesday that a dramatic loss of support is not unprecedented. In 1968, B.C. voters elected 16 Liberal MPs – including Mr. Anderson – but the caucus was reduced to four in 1972.

"It may be that [Liberal MPs] have been told – and maybe it's true – that this will all pass over and be forgotten by Christmas. I don't think that," Mr. Anderson said.

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But Ms. Fry cited a possibility that may work for the Liberals. "By the time 2019 comes around, people will judge our government based on a whole lot of other things than just this. I don't think governments win or lose based on one issue."