Mr Cain, 65, is a political outsider who rose unexpectedly to become the top contender to challenge Mitt Romney, the favourite of the Republican establishment for the nomination.
But he looks to be fading under the weight of the allegations that he sexually harassed women when he was president of the National Restaurant Association in the 1990s.
On Monday, Victor Jay Zuckerman, a former boyfriend of one of Mr Cain’s accusers, challenged the candidate’s credibility and his claims that he did not even know the woman, Sharon Bialek.
Mr Cain has denied any inappropriate behaviour and his attorney, Lin Wood, said he stood by his earlier statement that he does not remember Ms Bialek, the first of the four women to publicly claim unwanted sexual advances by the candidate.
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