We have seen this film before. The US Congress is threatening a trade war to punish “currency manipulators”, those it deems to be boosting their trade balance artificially at the expense of America. Ordinarily, a threat of legislative action like this folds in the face of a presidential veto. In the past this has resembled a good cop-bad cop routine. The White House would point to anger on Capitol Hill in order to persuade the likes of China and Japan to curb manipulation against the dollar.

This time, however, the number of Democrats and Republicans pledging their support for legislation looks unusually potent. President Barack Obama rightly insists that dealing with currency devaluation should be separated from matters of trade. He must hold firm both on the Trade Promotion Authority he seeks from Congress — that enables him to submit “fast track” deals to an up-or-down vote — as well as the Transpacific Partnership talks that are in their final stages. Both Atlantic and Pacific trade deals are at a crossroads. It would be a setback to global growth prospects if they were sabotaged by politics in the very country that initiated them.

However, Mr Obama should take nothing for granted. John Boehner, the Speaker of the US House of Representatives, has said he needs at least 50 Democratic votes to pass TPA. Yet 150 of the 188-strong Democratic caucus have already signed a petition opposing it. Unlike on previous rounds, they are joined by a growing number of Republicans who object to anything that increases Mr Obama’s authority.