The decision by the US Supreme Court to rule most of Donald Trump’s “liberation day” tariffs illegal will have far-ranging consequences for the president’s economic agenda. Although the administration will find other ways to increase tariffs, their usefulness as a weapon of economic warfare will be diminished. And the issue – among the most unpopular of the president’s economic policies – will cause him serious political damage.

Trump’s first move following the ruling has been to impose a 15% tariff on all imports. Imposed under a little-used law, the tariff rate is fixed and time-limited to 150 days before needing congressional approval. It would take only a few Republicans to block its extension. And the midterm elections are looming.

Using a flat-rate tariff means that some countries that settled earlier and got a better deal – including the UK – are now worse off, while others that had a higher tariff rate imposed on them have, at least for now, benefited. It also could mean that those that pledged to invest hundreds of billions in the US economy – including Japan and the EU – may now question whether their commitment still stands.

Trump’s ability to threaten instant retaliation to any country that crosses him will also be constrained by the other two legal routes he can use to raise tariffs. Both provisions would require time-consuming, detailed investigations into specific industries or countries, and rates once........

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