Donald Trump yesterday called on FBI director James Comey to “hang tough” in his investigation of Hillary Clinton, predicting the US would see the criminal trial of a sitting president if she wins next week’s election.

“He’s gotta hang tough because a lot of people want him to do the wrong thing,” Mr Trump said as the FBI chief’s decision to reveal new details about the bureau’s investigation into Mrs Clinton’s emails just days ahead of the election was sharply questioned by former top law officers and prosecutors.

With polls tightening, Mr Trump stepped up his assault on Mrs Clinton, telling a campaign rally in Michigan her election would “mire our country in a constitutional crisis that we cannot afford”. He predicted the US would have “a criminal trial for a sitting president” adding that Russian president Vladimir Putin would “sit back and laugh”.