It was unfortunate for US President Barack Obama that Rick Santorum this week chose to quit the race for the Republican nomination in the middle of his speech on the “Buffett tax”. But Mr Obama will give the millionaire tax plenty more airtime as the election approaches. The temptation to brand Mitt Romney as Mr One Per Cent is just too great.

On grounds of fairness, Mr Obama has a case. Named after Warren Buffett, the billionaire investor who famously pays a lower income tax than his secretary, it would impose a minimum 30 per cent tax on those earning more than $1m a year. Since it would include investment income, which is taxed at 15 per cent – less than half the top rate for incomes – it would roughly double the taxes that are paid by the likes of Messrs Buffett and Romney.

A large share of America’s wealthiest pay taxes of 15 per cent or less a year. This is lower than the average paid by the median household, which earns just $52,000 a year. At a time when politicians elsewhere, most notably Fran?ois Hollande, France’s Socialist presidential candidate, are pitching rates of up to 75 per cent, Mr Obama’s latest plan looks reasonable. On the substance, however, it amounts to less than it appears. First, it would only raise $47bn over a decade. That could rise to as high as $171bn if the Bush-era tax cuts are allowed to expire at the end of the year. That remains unlikely. Even then, the “Buffett tax” would reduce the annual budget deficit by less than 2 per cent.