Dave Walton is puzzled. The Iowan farmer cannot understand why most of the Democratic presidential contenders swarming the state are not talking about his big economic concern: the impact of the US-China trade war.

“I haven’t heard a lot of them speak much at all about trade,” says Mr Walton on his farm near the Mississippi river. “We’re an agriculture state and we depend on exports . . . To not talk about trade in Iowa is a big mistake.”

The tit-for-tat tariffs imposed by both sides over the past year have hit farmers in Iowa, which helped elect Donald Trump in 2016, and other agricultural states in the Midwest. Yet Democratic contenders have been slow to address the issue, partly because farmers tend to vote Republican. They are also concerned about appearing to be soft on China after Mr Trump’s tough stance on Beijing in the 2016 presidential race resonated with voters.