“WE DON’T win at trade,” Donald Trump frequently told supporters during his presidential campaign. America, he claimed, was “getting killed” by China, Mexico and Japan. On the campaign trail, pundits often dismissed such trade-bashing rhetoric. In particular, his comments about Japan—which by his reckoning sells America cars “by the millions” but buys “practically nothing” in return—were mocked as an outdated throwback to the 1980s. But with just weeks to go until he enters the White House, Mr Trump’s trade policies must be taken seriously. And they are no laughing matter.

Mr Trump has long argued that American firms are victims of a rigged trading system. To even the playing field, he has pledged to impose a 45% tariff on imports from China, which he said manipulates its currency to gain an unfair advantage. He has also promised to slap a 35% tariff on goods from Mexico, a destination for outsourced American jobs. The president-elect has vowed to renegotiate existing agreements like the North American Free Trade Agreement, which he called the “worst trade deal ever”, and to pull out of nascent deals like the Trans-Pacific Partnership and the Transatlantic Trade and Investment Partnership. Under a Trump administration, America could even conceivably withdraw from the 164-member World Trade Organisation, which Mr Trump has described as “a disaster”.

Such policies, should they be implemented, would be costly. America has free-trade agreements with 20 other states, which together represent a third of the country’s imported goods and nearly half of its exported goods. China is now America's largest trading partner. If Mr Trump follows through with his proposals, other economies would probably retaliate. A full-blown trade war would not be far off. The Peterson Institute for International Economics, a think-tank, reckons that under such conditions American private-sector employment would decline by 4.8m jobs, more than 4%, by 2019. This outcome would be most damaging to average American households on modest incomes—the very group whose interests Mr Trump claims to represent.