it's divide and conquer.
read the constitutional papers. it's baked into the system to get the black slaves and the white servants fighting each other instead of the landholding class, which, if you look into it, were largely literally european aristocrats. divide and conquer of the working class is the actual foundational principle america is founded on, not democracy or freedom or even property rights. if you want a secondary source, read howard zinn (or listen to a youtube video of chomsky paraphrasing zinn for you).
not only should the auto workers in canada and the united states be standing in solidarity with each other to fight the capitalists, but they should be trying to build solidarity with auto workers in mexico and china, too. the actual solution to the issues trump brings up, to the extent they are valid, lies in unionizing the workforce in china to eliminate their absolute advantage in labour costs, which will of course also benefit chinese workers, which american workers should see as a primary policy goal. stop competing. it's stupid. when we compete against each other, only the bankers win.