For a change, Donald Trump has been a model of restraint. Instead of stirring things up on Twitter, the Republican nominee struck the right tone after five police officers were killed in Dallas. Every American should be able to live in safety, he said — thus implicitly including black victims of police shootings. Now was the time for unity, prayer, love and leadership. His words could have been uttered by Hillary Clinton or Barack Obama. Having evoked every racial division in the book — and adding some pages of his own — could Mr Trump have turned a new leaf?

America should hope that he has. In political terms, 2016 is a racial tinderbox awaiting a match. On any sober measure, US society is more racially polarised at the end of Mr Obama’s term than at the start. A hundred days after he took office, 59 per cent of black Americans said US race relations were “generally good”. Six months before he leaves office that number has fallen to 34 per cent. Much of the pessimism stems from the spate of police shootings and the viral impact of incidents captured on video. But the disdain with which Mr Obama has been treated by his enemies has undoubtedly fed it.

It could hardly be otherwise. No president in US history has had his legitimacy questioned like Mr Obama. Mr Trump’s 2011 campaign to force Mr Obama to release his birth certificate proving that he was born in the US, not Kenya, petered out after the White House published the long-form document. But the echo of Mr Trump’s “birther” campaign carried by a broader crowd of Obama doubters was one of the reasons behind his decision to run for the presidency last year. It was a trial balloon in other words, which by Mr Trump’s measure was a success.