Autos are more obvious than the dairy industry, even though Trump claimed a couple of days ago that the reason there was an imminent deal is because Canada allegedly agreed to eliminate dairy tariffs? Trade negotiations have stalled almost certainly because Canada isn't actually willing to give up dairy tariffs.
Plausible.
The Quebec PM did say that if the dairy industry wasn't happy they wouldn't put US alcohol badk on the shelves, which would have effectively killed the whole thing.
While a factor I doubt this is the whole story though.
The dairy cartel is very strong in Canada (similar to the maple syrup one). They are deeply embedded in the government.
It'd be like trying to convince the US to reduce their Corn cartel. Except the corn industry keeps prices low, paid for by the American tax payer and demand mandates, and the Dairy cartel keeps prices high so consumers pay for the lack of competition.