The Canadian autoworkers union president, Jerry Dias, has been all over the media since the announcement. Today he's in Washington trying to pressure the U.S. gov't, UAW and Canada to work together.
He makes some compelling arguments such as:
1) This is not about electrification and the demand for Crossovers and trucks since the Oshawa plant is capable of building all models.
2) GM now will make only one model, the Equinox, at one plant remaining in Canada. They also make the Equinox in Mexico so if that model slows they can pull out of Canada completely.
3) If GM quits building in Canada all of the suppliers will close as well since there won't be enough work from the other company's plants, effectively shutting down the Canadian auto industry.
4) Mexico exports about 4 times as many GM cars as it buys for domestic use where Canada does the reverse.
5) Oshawa has always been one of GM's most award winning plants for quality and both Oshawa and Poletown are newer plants capable of producing all types of vehicles with minimal investment.

Dias would like a hold put on the new free trade agreement since the changes won't go into effect for 4 years. He would also like a 40% tariff or some other drastic action put on Mexican auto imports by the U.S. and Canada until the playing field is leveled.