Republican presidential nominee Donald Trump on Friday urged voters to check out an alleged 'sex tape' of a former Miss Universe who backs Hillary Clinton, provoking the Democrat to fire back that he is 'unhinged'.

In a pre-dawn tweet-storm of a sort guaranteed to grab attention, the Republican accused Clinton of helping Alicia Machado get US citizenship in order to exploit her during their presidential debate this week, the AFP reported.

The Venezuela-born Machado, who claims the billionaire bullied her mercilessly after she won the Trump-owned beauty pageant in 1996, has since shot to public attention.

'Did Crooked Hillary help disgusting (check out sex tape and past) Alicia M become a US citizen so she could use her in the debate?' Trump tweeted.

Clinton lashed out on the debate stage at Trump's history of abusive remarks about women, and assailed her rival for publicly humiliating Machado after she gained weight following her beauty pageant win.

Machado accuses Trump of verbally abusing her, calling her 'Miss Piggy' over her weight and 'Miss Housekeeping', apparently because she is Latina.

'Using Alicia M in the debate as a paragon of virtue justshows that Crooked Hillary suffers from BAD JUDGEMENT! Hillary was set up by a con,' Trump tweeted on Friday.

The Democrat lost little time in responding, tweeting that Trump's virulent stream of attacks bolstered her argument that he lacks the steadiness or temperament to be president.

'This is ... unhinged, even for Trump,' Clinton said in her own series of pointed tweets in response to his sex tape allegation.

'What kind of man stays up all night to smear a woman with lies and conspiracy theories?' she said.

'When something gets under Donald's thin skin, he lashes out and can't let go,' Clinton went on. 'This is dangerous for a president'.