Herman Cain is now trying to steady a campaign that has been rocked by controversy for the past 10 days

Republican presidential candidate Herman Cain has angrily denounced sexual harassment allegations against him and vowed they would not push him out of the 2012 race.

"Ain't going to happen," Cain told a news conference on Tuesday, referring to the chances of him giving up his presidential bid.

He described one of the accusers, Sharon Bialek, as a "troubled woman" produced by the "Democrat machine" to undermine his candidacy.

Cain said he had never seen Bialek before she held a news conference on Monday to lay out accusations that he made a crude sexual advance toward her when he was head of the National Restaurant Association in 1997.

He shrugged off a Reuters/Ipsos poll result showing his support declining among Republicans.

"It is natural that some voters will be turned off by the mere mention of the accusations. It is natural and it is expected," Cain told a news conference.

'Declining support' 

The poll released on Sunday showed the percentage of Republicans who viewed Cain favourably had dropped nine percentage points, to 57 per cent from 66 per cent a week ago.

The survey was the first evidence that sexual harassment claims against Cain have taken a toll on his presidential aspirations.

At least three other women have accused Cain of sexual harassment from his time as the restaurant industry's top lobbyist in the mid-1990s, but Bialek is the first one to go public.

Cain, 65, insists the claims are baseless and that he was wrongfully accused.

The former pizza company executive with no experience in political office, has been leading many opinion polls in the race to become the Republican presidential nominee to face Barack Obama in next year's presidential election.

The latest development came just before Republican candidates gather in Rochester, Michigan, on Wednesday night for the latest in a series of campaign debates.