D URING HIS lesser-known run for president, which began in 1999, Donald Trump proposed levying a wealth tax on Americans with more than $10m. He may soon find himself campaigning on the other side of the issue. That is because Democrats are lining up to find ways to tax the rich. Senator Elizabeth Warren, who wants Mr Trump’s job, has called for an annual levy of 2% on wealth above $50m and of 3% on wealth above $1bn. Alexandria Ocasio-Cortez, a prominent new left-wing congresswoman, has floated a top tax rate of 70% on the highest incomes.

In one way these proposals are a relief. Left-wing Democrats have plenty of ideas for new spending—Medicare for all, free college tuition, the “Green New Deal”—that would need funding. Mainly because America is ageing, but also boosted by Mr Trump’s unfunded tax cuts, the debt-to- GDP ratio is already expected to nearly double over the next 30 years. If a future Democratic administration creates new spending programmes while maintaining existing ones, higher taxes will be necessary.

If revenues are to rise, there are good grounds to look first to the rich. Mr Trump’s tax cuts are just the latest change to have made life at the top more splendorous. Between 1990 and 2015 the real income of the top 1% of households, after taxes and transfers, nearly doubled. Over the same period middle incomes grew by only about a third—and most of that was thanks to government intervention. Globalisation, technological change and ebbing competition have all helped the rich prosper in recent decades. Techno-prophets fear that inequality could soon worsen further, as algorithms replace workers en masse. Whether or not they are right, the disproportionate gains the rich have already enjoyed could justify raising new revenues from them.

Unfortunately, the proposed new schemes are poorly designed. Ms Warren’s takes aim at wealth inequality, which has also risen dramatically. It is legitimate to tax wealth. But Ms Warren’s levy would be crude, distorting and hard to enforce. A business owner making nominal annual returns of around 5% would see much of that wiped out, before accounting for existing taxes on capital. That prospect would squash investment and enterprise. Meanwhile, bureaucrats would repeatedly find themselves having to value billionaires’ art collections and other illiquid assets. Eight rich countries have scrapped their wealth taxes since 1990, often amid concerns about their economic and administrative costs. In 2017 only four levied them.