Donald Trump wants to slap punitive tariffs on China. Hillary Clinton opposes the 12-nation Trans-Pacific Partnership she once hailed as a gold standard for a new generation of free trade deals. Republicans are embracing Democrat demands for “fair” trade. The US, the architect of the open global system, is turning inwards.

The rest of the world should sit up. This is about more than the raw political emotions stirred by a US presidential race. The World Trade Organisation’s failed Doha Round saw the end of the multilateral trade liberalisation that gave us the globalised economy. The failure of the TPP would read the rites over the big plurilateral deals that promised an alternative. Free trade has been a powerful source of prosperity. It has lost political legitimacy. And not only in the US: European populists of left and right share the Trumpian disposition to throw up the barricades.

Optimists hope the protectionist turn in the US is cyclical. Things will get back to normal once the cacophony of the presidential contest subsides. Freed from the primary challenge of Bernie Sanders, Mrs Clinton, the most likely successor to President Barack Obama, will find a way to change her mind again. The TPP could yet be smuggled through Congress during the lame-duck interlude after November’s elections. Such is the line from Mr Obama’s White House and from a diminishing band of Republicans true to their free trade heritage.