Secret Service chief warns of 'elevated' threat to Clinton and Trump thanks to heated tone of the presidential race

He told CBS News the threat had grown on the campaign trail because offenders were getting braver.

Mr Clancy said: 'I would say the candidate threat picture is slightly elevated. People are bolder ... we have to be prepared for that.' 

The killing of five police officers at a Black Lives Matter demonstration in Dallas last week has also forced the authorities in Cleveland to finesse their security plans for the convention. 

Last month a British man, Michael Sandford, was arrested in Las Vegas at a Trump rally after trying to grab a police officer's gun.

Sandford reportedly said he planned to kill Trump but his mother, Lynne, said he needed psychiatric help.

In March another man was detained by Secret Service agents when he tried to approach Trump at a rally in Dayton, Ohio.

Thomas Dimassimo, 22, said he meant no harm to Trump and his only plan was to go up on stage, grab the mic and shout: 'Donald Trump is a racist.'

Mr Dimassimo later told CNN: 'If I can be unafraid enough to go take his podium away from him, then we all can be unafraid enough to not let this man walk into the White House.'

He has been charged with disorderly conduct and inducing panic.