TORONTO -- The union representing Canadian auto workers is bracing for a battle as it sets its sights on General Motors in the opening round of contract negotiations.

"Who do we perceive as the biggest challenge? It's General Motors," Unifor president Jerry Dias said during a news conference in Toronto.

"So if we're going to have a dust-up, we might as well have it immediately."

Unifor, which represents about 23,000 auto workers in Canada, announced Tuesday that it's selected GM as its target company in the ongoing contract talks with the so-called Detroit Three automakers.

That means that any agreement struck with GM will serve as a template for negotiations with the other two automakers, Ford and Fiat Chrysler -- a process referred to as "pattern bargaining."

Dias said GM was chosen because it poses the biggest challenge in terms of hammering out a deal.

He noted that new investment is direly needed at the company's engine and transmission facility in St. Catharines, Ont.

But, Dias added, he's even more concerned about the consolidated plant in Oshawa, Ont., and the possibility that GM may be planning to shutter both of its production lines at the roughly 100-year-old facility there.

The consolidated line, which produces the Chevrolet Equinox, was originally expected to close in 2005. After getting a new lease on life, the line is slated for closure next year.

Meanwhile, the flex line -- which produces the Chevrolet Impala, the Buick Regal and the Cadillac XTS -- has no product scheduled past 2019.

GM has previously said it won't make any promises about investing in its Canadian facilities or allocating any new products to its Oshawa plant until after an agreement with the union has been ratified.

"I'm here to tell you today that that is not going to happen," Dias said.

"We are not going to ratify an agreement with General Motors under any circumstance unless there is a commitment to our facilities. If that may be considered a line in the sand, so be it."

In an email, a spokeswoman for General Motors said the company remains "focused on working with Unifor to reach a mutually beneficial and competitive new agreement."

Unifor's contract with the Detroit Three automakers expires at midnight on Sept. 19.

Last month, members of the union voted in favour of strike action to back their contract demands, which include wage increases and securing more work to build vehicles.