As a Trump supporter, Malik Obama has already told US media the tycoon "can make America great again".

And in an interview with the New York Post, Mr Trump said Malik Obama "gets it far better than his brother".

The Kenyan-born US citizen, who is three years older than Barack, told the newspaper that he does not believe the women accusing the Republican of sexual assault. 

"Why didn't they come forward before?" said Malik Obama, who lives in Washington when he is not Kenya. 

The previous live debates were vitriolic affairs - and tonight's clash at the University of Nevada is likely to be just as tense.

Current polls put Mrs Clinton ahead leaving the billionaire businessman with a tough battle to win over undecided voters.

Ahead of the debate, Mr Trump has faced claims of sexual misconduct from a number of women, including a former contestant on the US version of 'The Apprentice'.

He has denied the claims, saying they are "outright lies" and his wife, Melania, has defended him in a recent interview accusing the claimants of being attention seekers making "damaging and unfair" accusations.

Mr Trump has also claimed the 8 November election will be rigged in favour of Mrs Clinton, saying: "The election is being rigged by corrupt media pushing completely false allegations and outright lies in an effort to elect her president."

This prompted Mr Obama to tell the Republican to "stop whining".

"If you start whining before the game's even over; 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," he said.

Mrs Clinton's campaign has also been hit by controversy over her use of her private email while Secretary of State, which Mr Trump has branded a scandal "worse than Watergate".

The 90-minute debate will be hosted by Fox News' Chris Wallace and will cover six topics - debt and entitlements, immigration, economy, the Supreme Court, foreign hotspots and fitness to be president.