Edward Stourton examines America's long history of resistance to free trade, and asks why it has again become such a potent political force. Donald Trump's most consistent policy has been opposition to free trade agreements which he sees as unfair, particularly with China. On the Democratic side, Bernie Sanders has been equally opposed, if for different reasons, while Hillary Clinton has had to tack away from her previous support for free trade pacts. Edward looks back to debates from the 19th century to the 1990s to shed new light on these forces. And he asks whether the protectionist impulse is a natural reaction to globalisation's wrenching changes. Producer: Smita Patel.