Obama: America at risk if Trump wins

President Barack Obama has delivered an impassioned plea for voters to turn out for Hillary Clinton, warning that not just his legacy but also the country is at risk if Donald Trump becomes president.

Obama told voters in the crucial swing state North Carolina - which he lost in 2012 - that the future of the country rested in their hands as he launched a blistering attack on Trump, describing him as "uniquely unqualified" to be commander-in-chief.

“I ran against John McCain, I ran against Mitt Romney - I thought I would be a better president but I never thought that the Republic was at risk if they were elected,” Obama warned.

He urged the large crowd to look past "the noise" of the ugly election campaign and to elect Clinton as the first female president.

"Choose hope! Choose hope! Choose hope! Vote and if you do, we will elect Hillary Clinton as the next president of the United States."

Obama, whose final huge task as president is to ensure the preservation of the legacy of his eight-year term, has been campaigning relentlessly for his former secretary of state.

Clinton looked on course for the White House before the FBI announced a fresh inquiry into her email use, throwing the race into disarray and handing her rival a lifeline.

Obama earlier criticised the way the FBI revealed the investigation last Friday, saying the bureau should not operate on the basis of "innuendos" or "incomplete information".

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In a further worrying sign for Clinton - who on Tuesday night vented her frustration at a heckler who called her husband a rapist - early turnout for African-American voters in swing states has dropped since 2012.

Obama has admitted that the voting demographic "is not as solid as it needs to be” and used his speech to galvanise the vote and millennials, urging them not to waste their chance of influencing history on November 8.

"It's not often that you can move the arc of history - don't let that chance slip away."