Paul Krugman used his column today to criticize Donald Trump's tariffs for everything policy. While he provides a useful historical account of the abuse of tariffs in the 1920s, he left out one important reason why tariffs can be pernicious, especially in the hands of someone like Donald Trump.
Trump obviously views the presidency as a position to use for rewarding friends, punishing foes, and enriching his family. Tariffs give him a great opportunity to do this.
Not only can he put a tariff on an essential imported input for a company that criticizes him, but he can also give exemptions to tariffs (they always have exemptions) for his friends. The NYT had a good piece on the issuing of exemptions last year.
The potential for abuse of the discretion associated with tariff policy is a cause for concern in general. However, with a president and a Republican party that have corruption as a guiding principle, this discretion is a very big deal. We should absolutely assume that Trump will abuse his tariff authority. It would probably never even occur to him that it is wrong.