As polls have turned against him, Donald Trump’s allies have sometimes placed weight on the existence of a shy tribe of supporters who are reluctant to declare themselves and whose invisibility in the polls means Hillary Clinton’s advantage is being overstated.

The problem for the New York businessman? Pollsters have looked far and wide and deep into their methodology and they say they have struggled to unearth evidence of the shy Trump voter theory advocated among others by Kellyanne Conway, Mr Trump’s campaign manager.

The debate is just one of a legion of questions hanging over the accuracy of opinion polls heading into a razor-thin contest. Recent opinion polling mishaps in the UK, Israel and Spain have raised the stakes for pollsters heading into November, as has Mr Trump’s heavy emphasis on polls in his public statements — at least when they are going his way.