US Politics

In simple terms, they make imports more expensive.

Basically tariffs are a good idea for a developing country that needs to nurture its own industries to allow them to get to the point where they are competitive internationally. At that point you suddenly want everyone to get rid of tariffs so your now world-leading industries can access the global market.

When the US became independent the British said, 'let's have free trade - you send us the raw materials, we'll make stuff and you buy it.' The Americans said 'Fuck that, we will put tariffs on everything and develop our own manufacturing.'

After the war, the US set up the IMF and World Bank, who said to Europe and all the developing countries 'Let's all have free markets so your people can buy our stuff more cheaply'. The developing countries said 'but don't developing countries need tariff protection for their emerging industries to develop, or else they will be trapped as permanent exporters of raw materials at low margins?' WB and IMF said 'that's right, but if you don't want your people to starve / have sanctions / be invaded / have coups / etc - no tariffs.' Europe and dev world said 'ok then'

Now, the US can no longer compete internationally in many markets, tariffs suddenly look like a less bad idea. But it would mean that American manufacturing would gardually become less and less competitive globally (think cars) and only focus on their domestic market. In many ways it would be similar to the 1930s. It would be bad for Americans, but worse for virtually everyone else.