Canada's auto union has been aggressively lobbying for GM to assign product to its Oshawa Assembly Plant. GM feels, though, that the Unifor's latest strategy goes too far.

The automaker threatened to sue Unifor over an ad it decided to run during the Canadian broadcast of the Super Bowl anyway.

The ad brings up the $10.8 billion loan that GM took from the Canadian government following the economic crash in 2008. Unifor then criticizes GM for assigning more vehicles to Mexican plants.

GM takes umbrage at the first criticism, pointing out that it repaid that loan from the Canadian government.

"We cannot condone purposely misleading the Canadian public," GM told the Detroit News on Sunday. "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 over $100 billion to the Canadian economy including $8 billion invested into worker pensions."

Unifor has been one of GM's biggest critics since GM announced that it would not be assigning it would effectively be shutting down the Oshawa plant, along with the Detroit-Hamtramck and Lordstown assembly plant, late last year.

Along with the ad, the union has organized walk-outs at the Oshawa plant and protested GM outside the Detroit Auto Show.