WASHINGTON - Republican presidential contender Herman Cain plowed ahead Wednesday amid mounting allegations of sexual misconduct including an extramarital affair. But he acknowledged the toll was rising and said he would decide by next week whether to drop out of the Republican race.

Cain spent a month battling several sexual harassment accusations, which took a toll on both his standing in polls and, supporters say, his fundraising.

The latest furor came Monday when businesswoman Ginger White, 46, said she and Cain had maintained a consensual sexual relationship spanning more than a decade and ended this year before he became a candidate.

Prominent conservatives who rushed to his defence when the first allegations of inappropriate sexual behaviour surfaced were all but silent after the affair accusation. The turmoil comes just five weeks before the first votes are cast in the state-by-state march to the nomination.

Former House Speaker Gingrich has been the beneficiary -- in polls, at least -- of Cain's slide in the month since it was disclosed that the National Restaurant Association paid settlements to two women who claimed Cain sexually harassed them while he was president of the organization. A third woman told The Associated Press that Cain made inappropriate sexual advances but that she didn't file a complaint. A fourth woman also stepped forward to accuse Cain of groping her in a car in 1997.

Cain said Wednesday that he would exit the race if the price proved too high and he would make a decision by the middle of next week at the latest. He has denied all wrongdoing.

But for now the campaign is continuing. Aides to the ex-restaurant executive, lobbyist and motivational speaker were moving ahead with plans for events in around the country and prepared to launch a fresh round of TV ads in Iowa, where the first state contest on the road to the nomination will be held Jan. 3.

Cain, on a one-day bus tour of Ohio, insisted he was seeing "a groundswell of positive support" after the latest allegation threatening his campaign. Still, he acknowledged "we are reassessing and we are re-evaluating" in light of the woman's account, which followed accusations of sexual harassment by other women in recent weeks.

In an interview on Fox News late Wednesday, Cain said the controversy has taken an "emotional toll" on his wife, Gloria.

"I've got to think about my family first, especially my wife," Cain said. "This is why we are reassessing."

At his campaign stops, he renewed what has become a familiar defence: that he is the victim of attacks by liberals and the establishment, who are threatened by his outsider appeal.

"They want you to believe that with another character assassination on me that I will drop out," a defiant Cain told a crowd of about 200 in Dayton. The boisterous crowd greeted him with shouts of "no!" and "boo!"

"One of the reasons they are trying to shoot me down and tear me down is the strength of my message that resonates with the American people," he said.

Cain drew enthusiastic crowds in three appearances in the state. Though there were signs that some in early voting Iowa and New Hampshire were reconsidering their support -- and political veterans were beginning to suggest his campaign's days were numbered -- some backers here said they were deeply skeptical of the mounting allegations.

Among political operatives, however, the perception was setting in that Cain's troubles were causing irreparable harm to his bid.

"I don't see how they walk away from the damage that's been done and emerge as a viable primary candidate," said Rick Wilson, a longtime Republican consultant. "All these things about Herman Cain keep coming out drip, drip, drip, and they're not handling it well. And now conservative Republicans have another place to go: Newt Gingrich."

Dan McLagan, a veteran Republican strategist, agreed, saying: "Cain is like a zombie at this point: he's dead but he does not appear to have noticed and has kept on walking. His support is all moving to Gingrich and, at some point, he's going to look back and see that he is grand marshal of a one-man parade."