Last month, as the US midterm elections approached, Deutsche Bank analysts released a calculation that should have made American voters wince. It shows that the US government currently pays $1.43bn each day (yes, day) to service its public debt — 10 times more than any other G7 country (Italy is a distant second in this grim league).

This is striking, even allowing for the size of the American economy. But what is doubly thought-provoking is that this $1bn bill has materialised when interest rates are still fairly low by historical standards. And that invites a crucial question for the US Congress: what will happen to that debt, and servicing costs, if (or when) interest rates climb to a more normal level?

Until recently, neither investors nor voters seemed to care particularly. After all, asset managers have flocked to buy US treasuries in recent years, even as America’s debt pile swelled above $15tn. And those once-feared bond vigilantes seemed all but dead last year when President Donald Trump’s government announced massive tax cuts, further increasing debt.