WASHINGTON - Republican presidential candidate Herman Cain struggled on two fronts in the party's topsy-turvy race for the nomination as he stumbled badly when asked about the Libyan civil war and yet another witness came forward to bulk up allegations he sexually harassed women.

Cain is a political novice who rose unexpectedly to become the top contender to challenge Mitt Romney, the favourite of the Republican establishment, but he looks to be fading under the weight of the allegations he sexually harassed women when he was president of the National Restaurant Association in the 1990s.

Those troubles have been compounded by what appears to be a thin knowledge of U.S. foreign policy.

And some polls show Cain drifting downward, ceding his place to former House of Representatives Speaker Newt Gingrich.

On Monday, Victor Jay Zuckerman, a former boyfriend of one of Cain's accusers, challenged the candidate's credibility and his claims that he didn't even know the woman, Sharon Bialek.

Cain has denied any inappropriate behaviour and his attorney, Lin Wood, said Cain stood by his earlier statement that he does not remember Bialek, the first of the four women to publicly claim unwanted sexual advances by the candidate.

Zuckerman, while not a witness to Cain's alleged behaviour, told a news conference that he and Bialek spent considerable time with Cain at a NRA convention dinner and party afterward. He confirmed Bialek's contention that she later went to Washington to seek Cain's help with employment.

Zuckerman stood by Bialek's assertion that Cain had behaved poorly and insisted Cain was not telling the truth about not knowing her.

Also Monday, in a video interview with the Milwaukee Journal Sentinel newspaper, Cain bungled a question about whether he agreed with President Barack Obama's handling of the Libyan civil war. Cain searched for an answer, then after a long pause asked if his questioners were asking if Obama had supported taking action against the country's dictator, Moammar Gadhafi.

At that point, Cain said he did not agree for "the following reason." Then he said, "Um, nope that's a different one," waving his had as if to dismiss the thought.

After another long pause he said, "I gotta go back, see, got all this stuff twirling around in my head."

The video made it to the internet and gained a huge audience almost at once.

Less than two months remain until the state of Iowa holds the leadoff selection process -- a complicated series of local caucus votes.

Cain has vowed to stay in the race and now is using his wife of 43 years to defend him.

Hours after Zuckerman stepped forward to give his account, an interview with Gloria Cain aired on Fox News Channel. She defended her husband as the turmoil over the claims of sexual wrongdoing stretched into a third week.

"I know the person that he is, and I know that the person that they were talking about, I don't know who that person is. We've been married for 43 years," she said in an interview taped over the weekend.