A viewer of the US presidential debate on Tuesday got an uncomfortable sense of how exhausting a White House campaign can be. Each candidate had the chance to take a rhetorical bludgeon to his rival, but lacked the energy or wit to do so. When Mitt Romney promised to help the middle class by cutting taxes on interest, dividends and capital gains, Barack Obama could have mocked him for peddling a plan for the rich. Instead, the president changed the subject. Mr Obama’s aides, meanwhile, have left a long trail of video evidence that they sought, for reasons still unclear, to misrepresent a terrorist attack that killed the US ambassador in Benghazi on September 11 as a protest that got out of hand. Mr Romney sputtered a bit about the attacks, but so incoherently that Mr Obama seemed to be the only person with the slightest idea what he was talking about. Willy-nilly, presidents must make most of their critical decisions in this frame of mind: exhausted and with too much information rattling around in their heads.

There was one subject, however, on which the candidates were clear-headed and wholeheartedly in accord. They are both keen to “crack down” on China. Mr Romney accuses China of depressing its currency, the renminbi, to maintain its current account surplus with the US. “On day one,” he said, “I will label China a currency manipulator” – a formality that would allow him to take certain remedial actions. Mr Obama did not reject Mr Romney’s complaint in principle. He said only that he had found effective ways to get China to drive up the value of the renminbi by 11 per cent. He also accused Mr Romney of having “invested in companies that were pioneers of outsourcing to China”.

Both candidates are retreating from free trade in the name of free trade. They risk making hard-to-break promises without realising it. Mr Romney had used his “day one” line on the campaign trail in post-industrial Ohio in September, but speeches in the heartland are more easily forgotten than nationally televised debates. Mr Obama is right that the renminbi has fallen in recent years, although this is probably more because of a downturn-induced decline in demand for China’s exports than anything his Treasury department has done. Charles Dumas of Lombard Street Research has argued lately that by ignoring inflation in China’s labour markets and export sector, we overestimate China’s competitiveness.