"Stop whining," President Barack Obama rebuked Donald Trump, speaking out as seldom before on next month's US election and chiding the Republican for sowing suspicion about the integrity of America's presidential vote.

On a day of discord between the White House and the Republican nominee, Trump invited two eyebrow-raising guests to today's presidential debate: Obama's estranged half-brother and the mother of an American official who died in Benghazi, Libya.

Malik Obama, the President's half-brother, will be in Las Vegas supporting Trump.

Earlier yesterday, Obama lashed out at Trump, saying that the Republican nominee's insistence that the election was being rigged undermines the country's democratic traditions. "You start whining before the game is even over?" Obama asked at a Rose Garden news conference. "If whenever things are going badly for you and you lose you start blaming somebody else - then you don't have what it takes to be in this job."

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Obama added: "I have never seen in my lifetime or in modern political history any presidential candidate trying to discredit the elections and the election process before votes have even taken place."

"There is no serious person out there who would suggest somehow that you could even rig America's elections, in part, because they are so decentralised. And so I'd invite Mr Trump to stop whining and go try to make his case to get votes."

A Gallup poll showed yesterday that Obama's approval rating had hit a high of 54 per cent for the first time since January 2013.

Trump is also trying to get in rival Hillary Clinton's head by inviting Pat Smith, whose son Sean died in the 2012 attack on a diplomatic compound in Benghazi. Smith has been a vocal critic of the Democrat Clinton and delivered a speech at the Republican Convention in July. Republicans have blamed Clinton for the tragic deaths of four American in Benghazi, though several congressional investigations have not found any evidence of it.

Trump shocked reporters and political observers last week in St Louis when, hours before the second debate, he held a press conference with several women who had accused former President Bill Clinton of sexual abuse. The Clintons have long denied the accusations. The Trump campaign had also sought to embarrass the Clintons by inviting several of the women to sit in the family box near the debate stage. That plan was ultimately barred by debate officials.

For her part, Clinton is bringing frequent Trump critic and billionaire Mark Cuban and Hewlett Packard Enterprise CEO Meg Whitman.

- Washington Post, AP