Politicians have often struggled to explain how governments borrow money to fund their spending. The implications of higher or lower borrowing are also difficult to assess when the figures run into hundreds of billions of pounds.

The temptation is to simplify the arguments by comparing the nation’s finances to a household budget or a credit card. The shadow chancellor, Rachel Reeves, has talked about the Conservatives “maxing out the credit card”, and Keir Starmer and Rishi Sunak have…