First, I do agree that tariffs are almost always economically harmful, no matter what their political effects.
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But the situation is not as clear as you paint it. If the cost of a parts being imported by GM goes up by $200, ONE option is for GM to simply charge more to the consumer. Just as likely is that GM's profits would go down as GM is forced to absorb the cost, since the amount that the consumer is willing to pay for the product hasn't change. After all, if the consumer was willing to pay $500 more, GM would be charging $500 more and pocketing the additional profits.
A particular product is bought from China because it makes economic sense to do so. Sure, it is possible that with a 20% tax, China's product is the still the best choice. More likely is that it is not.
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WHAT HAS HAPPENED SO FAR?
There have been lots of tariffs (taxes) put on Chinese goods by Trump, and retaliatory tariffs put on us by China. The result is that we are buying less from China, and they are buying less from us. The deficit with China is way down, just as Trump wanted. HOWEVER, the total trade deficit is up. Consumers are actually buying MORE imported goods, rather than less, just not from China.
I'm fine with this result. Sure, some of our industries are hurt. There will be a transition as our companies become less dependent on sales to China. Again, this is a good thing, and Trump, like politicians in the past, will subsidize the farmers who are unable to compete on the world market.
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AS AN ASIDE
I certainly believe that Trump needs to reduce or eliminate tariffs on aluminum and steel, or certainly give waivers to China, Mexico and some of the EU countries. We are in an economic war against China, who cheats in their economic dealings. So, we should be in such a "war". There is great support in Congress for our efforts against China. Trump needs to stop fighting with our economy allies.
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I wonder if folks realize that the importer is the one who pays the duty, then passes off the cost to the consumer. So GM orders parts they need from China, then has to pay the duty, and they pass the cost of that duty onto the consumer.