The war between Canada's autoworker union and General Motors intensified over the weekend, with the automaker threatening to sue over a union commercial critical of GM set to air in Canada during Sunday's Super Bowl.

On Friday, lawyers for GM wrote to Unifor, the Canadian union, with a “demand” that Unifor “cease and desist from any further publication (in any form and media whatsoever) of the advertisement.”

The union said it would not comply with GM's demand.

Unifor has been waging a publicity campaign against GM since late November, when the automaker announced it would indefinitely idle the Oshawa Assembly plant in Ontario and four other facilities in North America this year. In Michigan, Detroit-Hamtramck and Warren Transmission are on that list. In total, some 6,000 factory jobs are jeopardy.

Unifor's commercial is called “GM leaves Canadians Out In the Cold.” It appears on YouTube, on Unifor's Facebook page and was scheduled to air throughout Canada during the Super Bowl. It asserts that Canadians have been loyal to GM and now the company has "forgotten our generosity.”

“We stand by the belief that if GM wants to sell here then it needs to build here, and we will not be intimidated from sharing that message with Canadians in this ad,” said Unifor President Jerry Dias.

The ad cites Canadian aid to the automaker as it emerged from bankruptcy in 2010, providing $10.8 billion, which the union says is $300 per Canadian citizen.

The letter, addressed to Dias and copied to Canadian media companies from Gowling WLG, a Toronto law firm, tells Dias, "You know this is untrue." It takes particular issue with the union's assertion of "greed" by GM, which has rejected Unifor's proposals to save the Oshawa plant.

The law firm says Canada "provided temporary monetary assistance to the newly restructured GM entity and GM Canada in the form of an interest-bearing loan. This loan was fully paid back with interest to the Canadian government."

"Repayment of both the principal loan and all interest due was accomplished in the form of both cash and equity shares in the new GM, which shares were obtained at fair value. The Canadian Government later decided, on its own volition, to sell its shares in GM many years ago on that same open market."

The U.S. government also became a major shareholder in post-bankruptcy GM, providing $50 billion in initial aid, and ultimately lost about $10.5 billion after it sold its shares.

Mexican factories

Unifor and the UAW have complained that GM has shifted vehicle production to plants abroad, namely Mexico, where wages are cheaper. Both unions have called for consumers to not buy GM vehicles that have a vehicle identification number starting in "3" because it means it was built in Mexico.

Of particular irritation is GM's plan to produce the new Chevrolet Blazer SUV in Mexico. The plants being closed produced mainly sedans as consumers shift to pickups and SUVs.

More:'We're not going away,' Canadian autoworkers say in GM protest

More:UAW joins Canada autoworkers' call to boycott GM cars made in Mexico

GM confirmed Sunday that it did authorize the legal demand that Unifor cease showing the commercial.

“While GM respects Unifor’s rights to protest, we cannot condone purposely misleading the Canadian public," said GM spokesman Pat Morrissey. "The new Unifor advertisement scheduled to air during the Super Bowl is misleading and inaccurate.

"Unifor knows that GM Canada repaid its 2009 loans in full, and that the restructured GM fulfilled all the terms of its agreements with the Canadian government many years ago."

Since 2009, GM Canada has contributed more than $100 billion to the Canadian economy including $8 billion invested into worker pensions, Morrissey said.

He said he knew of no plans to make similar demands of the UAW, which also is protesting the plant closures but has not been as aggressive as Unifor.

More:Canadian union slams GM in ads over Oshawa plant closing

An additional commercial to debut this weekend encourages consumers to take an online pledge to boycott GM vehicles made in Mexico if the company fails to maintain production in Oshawa and other Canadian facilities.

GM's Toronto law firm said if the ad continued, the company would consider suing under Canadian Trademark and Competition acts, and libel law.

Contact Jamie L. LaReau: 313-222-2149 or jlareau@freepress.com