The federal New Democrats will attempt to score points against the majority Liberal government when Parliament resumes this fall by declaring the preservation of public health care to be their primary cause.

NDP officials hope the line of attack will resonate with Canadians at a time when a British Columbia doctor is challenging some of the restrictions of the public system, and as the Liberals look to write a new health accord with the provinces.

"I think Canadians may not be aware of the serious threats that confront medicare," Don Davies, the NDP health critic, said on Wednesday in a telephone interview. "We've got to strongly be on the Liberal government's case to make sure that they actually increase funding, and make sure that the federal government is increasing resources to the provinces."

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The New Democrats, who were reduced to third-party status in the 2015 election after occupying the offices of the official opposition, will have to work hard to stay relevant in Parliament, where the Liberals – who campaigned on many of the same issues – hold the majority of seats.

Its members voted earlier this year to turf their leader, Tom Mulcair, who is staying on until his replacement is chosen the fall of 2017. Mr. Mulcair has been a strong voice in the Commons and is routinely commended for his "prosecutorial" style. But he needs an issue that will capture the interests of voters.

The suit launched by Brian Day, the medical director of a private health-care clinic in Vancouver, has given the NDP the opportunity to decry what it calls the "creeping privatization" of medicare. Dr. Day is challenging B.C. regulations that ban private care for medically necessary services.

"That litigation threatens one of the core underpinnings of the Canadian medicare system, which is that everybody should have equal access to medical services without regard to your ability to pay," Mr. Davies said.

The Liberal government has promised to negotiate a new health accord with the provinces and territories that will include a long-term agreement on funding.

In 2004, then-prime minister Paul Martin signed a 10-year, $41-billion accord that gave the provinces and territories a 6-per-cent annual increase in federal health-care dollars. When that accord expired in 2014, former prime minister Stephen Harper extended the 6-per-cent annual increase for three more years, but said the escalator would drop in 2017 to 3 per cent a year (or the equivalent of GDP growth, whichever is greater).

The New Democrats say cutting the size of the increase will be a $1.1-billion-per-year blow to the health-care system starting in January.

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"If the federal government reduces its funding to the provinces, and the provinces don't have as much money to fund the front-line services, then wait times get longer, the service delivery gets poorer and that opens the argument for people like Dr. Brian Day to misdiagnose the problem," Mr. Davies said.

The NDP has created a web page where Canadians can urge the government to stop what New Democrats say are cuts to the system.

The Liberals say there is no need to worry about the state of public health care while they are in power.

"A strong public health-care system, grounded in the principles of the Canada Health Act, is key to a fair and just society, and essential to maintaining and growing a strong and healthy middle class," Health Minister Jane Philpott said in an e-mail. "Our government is committed to working with provincial and territorial governments to improve our health-care system, and the health of all Canadians."

As New Democrat MPs prepare for the fall session, some of the rank-and-file members are questioning the decision to eject Mr. Mulcair. No one has, so far, stepped forward as a potential successor, and former NDP candidate Dale Jackaman has launched a Facebook page called "Bring Back Tom Mulcair."

Mr. Jackaman said he and others are angry at the way Mr. Mulcair has been treated. "I think we are looking at being an opposition party for the next few years, probably for a term or two, and we need our best political gunslinger," Mr. Jackaman said.

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But Mr. Mulcair suggested he is not interested in staying on.

"I was very touched," he said in an e-mail of the Facebook page. "For the past ten years I have dedicated myself, heart and soul, to the party. I have agreed to the request from caucus and the party to stay on until a new leader has been elected and that's what I'll do."