Here is how Donald Trump can win the presidency

You will struggle to find a group of people more profoundly pessimistic and bitter than the working-class voters of the US. Every time Donald Trump attacks the global political and economic elite in language that makes the establishment shudder, these voters cheer.

As happened in Britain before the EU referendum, too many commentators have underestimated the depth of discontent. Seventy per cent of voters believe that America is on the wrong track and more than half say that they are “struggling to get by”. Most alarmingly for a country that prides itself on intergenerational improvement, 58 per cent believe that the next generation will be even worse off than they are.

Are there enough angry voters to propel Mr Trump to the presidency? The simple answer is yes. Even before it was revealed that Hillary Clinton, his Democratic rival, is suffering from pneumonia, the race had tightened. Mr Trump has closed the 8 point post-convention gap between him and his opponent and is three points behind her nationally.

He has pulled level in Florida, the largest of the vital swing states. Several other states that were trending towards Mrs Clinton, such as Ohio, have moved back into the “too close to call” category.

Yet if the race continues on its current course, Mr Trump will still lose because Mrs Clinton is ahead or tied in every state that went for President Barack Obama in 2008 and 2012. Nevertheless, her negatives continue to rise. She is regarded unfavourably by about 55 per cent of the electorate. No Democratic nominee in modern history has ever had an unfavourability rating that high.

After a quarter of a century of questionable candour, there is little that Mrs Clinton can do to repair her credibility with voters at this late stage. But there is plenty she can do to capitalise on Mr Trump’s unforced errors.

For the Republicans, this campaign has been a tale of two Trumps. The “good Trump” has an outsider anti-establishment appeal that voters respond to. The “bad Trump” traffics in gratuitous insults. It is no surprise that he has earned a 60 per cent unfavourability rating, the highest of any nominee of either party in modern history. If Mrs Clinton is too scripted, Mr Trump is not scripted enough. The more he talks off the cuff, the worse he does. He should be a vessel for America’s anger, not an outlet for his own personal rage.

In the Republican primary, Mr Trump was able to insult his way to the nomination. But he cannot do this in the general election. Moderates, independents and conservative Democrats certainly want a leader who speaks the truth but not at the expense of civility and respect. Americans want Mr Trump to be a truth-teller and to be presidential.

Next he needs to sharpen his message by adding specifics. He is at his best when he is channelling popular anger and the demand for change. On economic issues, significant majorities of Americans back Mr Trump’s ideas. They want a meaningful reduction in government spending. They want to take control of their healthcare. They want charter schools and the right to hold teachers accountable. You do not hear enough of this from Mr Trump. His persona is drowning out his policies.

Finally, Mr Trump needs to learn the right lessons from the Brexit vote. What makes the UK’s referendum relevant to the US election is not the result, it is the resentment. No one wants to be lectured or hectored by professional politicians they dislike and distrust. The same emotional tide that swept the Leave campaign to victory is rolling across the US. Mr Trump is at his best when he rides that wave rather than trying to claim it as his own. If he can do that, while being civil and respectful, he could be the next president of America.

The writer is a pollster and communications adviser