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I suspect we would see the American domestic political equivalent of World War III before all those foreign-owned and/or U.S. owned auto and steel firms would put themselves in a position where they could not only be unionized, but dragged "up" to the position of the UAW and similar high end unions.
That's how nearly all unions were established in the first place, all over the world. They were practically slave revolts.
Try googling "union history" if you're really interested.
Modern US and European worker pay is as high as it is, and working conditions made tolerable, essentially because people died (for your paycheck) a hundred years ago and more.
Unions are expected to be forced to collapse eventually, one way or another, and that will be the end of the US middle class. Employment dynamics being what they are, the unions are basically all that prevent US corporations from turning the US into a third-world country. Even though they represent only about 10% of US workers, the unions effectively put upward pressure on all wages by forcing most employers to pay to compete for preferred workers.