For what is surely the first time ever, conservative talk show host Bill O'Reilly has agreed with comments from US President Barack Obama.

"Stop whining! Stop that," O'Reilly told The Late Show host Stephen Colbert when asked about Republican president candidate Donald Trump's claims that the current election is 'rigged'.

"That's whining," O'Reilly, a pundit on Fox, added as he spoke ahead of the third presidential debates tonight (October 19). "He should have a little buzzer that every time he whines it's like 'baaaa!'"

Trump has repeated and increased his claims over recent days that the election is rigged, after more than nine women came forward to accuse him of sexual assault.

"I'd invite Mr Trump to stop whining and go try to make his case to win votes," said Obama during a press conference on Tuesday, addressing those claims.

O'Reilly said that if he were Hillary Clinton that he would ignore Trump and make her case directly to America. "The more time she spends on him, it just becomes vaudeville," he said.

On Trump, he said: "I want specifics. You're going to take their oil? How are you going to take their oil?"

The third and final presidential debate takes place tonight at the University of Nevada in Las Vegas. The debate will be moderated by Fox News anchor Chris Wallace. Americans go to the polls on November 8.