In June 1930, the Smoot-Hawley Tariff Act increased U.S. tariffs on agricultural imports and more than 20,000 imported goods. The tariffs imposed were the second-highest in American history. The goal was to protect American farmers who were most affected by the Great Depression. However, it raised the prices of food and other items.
Other countries retaliated with their respective tariff hikes, forcing global trade to decline by 65%. Economists believe that the Smoot–Hawley Tariff Act was one of the principal causes of the economic depression. The legislation highlighted how dangerous protectionist trade policies are for the world economy. Afterward, most countries promoted free trade agreements that support fair trade for all.
In the past, the Republicans have generally disliked even a whiff of protectionism. Which makes what Trump insists on especially odd. But I think he does it simply because he isn't very bright, and doesn't grasp the concepts involved. He doesn't care that ordinary consumers foot the bill.