* Claim of affair follows series of missteps for Cain
* Atlanta woman's allegation 'fabricated', Cain says
* Cain pleads with reporters, 'Stand with me' (Adds Cain's email to supporters)
By John Whitesides and Sam Youngman
WASHINGTON, Nov 29 (Reuters) - Republican presidential contender Herman Cain told aides on Tuesday he would reassess the viability of his struggling campaign after an Atlanta woman accused him of conducting a 13-year extramarital affair.
The allegations could be the end of the political line for Cain, who led the Republican White House race barely more than a month ago but has nosedived in polls after a series of sexual harassment charges and campaign missteps.
Cain, 65, denies he had an affair with businesswoman Ginger White, who told an Atlanta television station that her on-and-off relationship with Cain began in the mid-1990s.
In a fundraising email to supporters entitled "Stand With Me," Cain called White a "troubled Atlanta businesswoman" who is pushing a "fabricated, unsubstantiated story."
"I do know Ms. White. I have helped her financially at times over the past few years, just as I have helped many friends and acquaintances throughout the years. I thought Ms. White was a friend in need of a supportive hand to better her life," he wrote.
The former pizza executive told aides in a conference call transcribed by the conservative magazine National Review that he would gauge the impact of the charges over the next few days and see if they created a cloud of doubt in supporters' minds.
"Obviously, this is cause for reassessment," Cain told staff members, adding he would continue his campaign schedule over the next few days.
"The public will have to decide whether they believe her or whether they believe me. That's why we're going to give it time, to see what type of response we get from our supporters," he said.
The news began to take a toll on Cain in New Hampshire, where WMUR television reported that two Cain supporters, state representatives William Panek and Sam Cataldo, will now back Newt Gingrich.
LITTLE TIME BEFORE IOWA
The allegation of an affair comes five weeks before Iowa kicks off the Republican battle for the right to challenge Democratic President Barack Obama, giving Cain little time to resurrect a once high-flying campaign.
In addition to sexual harassment allegations by four women, Cain also has stumbled on the campaign trail. He raised alarm among conservatives with confusing comments about abortion and badly fumbled a question on Libya policy.
His fundraising also appears to have stalled. Aides who crowed about his strong surge in donations in October have declined to answer questions about his cash flow in recent days.
"It's over for Cain. On the heels of everything else, I don't see how you come back from this," Republican strategist Rich Galen said.
Cain's difficulties have opened the door for Gingrich, the former House of Representatives speaker who has zoomed to the top of opinion polls with the support of conservatives hunting for an alternative to the more moderate Mitt Romney in the Republican race.
Conservatives have failed to coalesce around a single candidate this year as several contenders - first U.S. Representative Michele Bachmann, then Texas Governor Rick Perry and then Cain - have risen in polls only to fall back.
'ONE MORE THING'
"I don't see a path to victory for Cain anymore," said Ryan Rhodes, founder of the Iowa Tea Party. "He was already looking at the end of the line, but this is one more thing to finish him."
Gingrich has seen his poll numbers rise as Cain's crumbled. Support for Romney, the former Massachusetts governor, has held relatively steady in the mid-20s in polls for much of the year.
Gingrich, who is in his third marriage, told CBS News the accusations did not disqualify Cain and he should be given a chance to recover.
"Remember, I was supposed to be dead in June and July," said Gingrich, who hit bottom in opinion polls after a mass staff exodus over the summer. "I am not going to go around and declare anybody else dead."
The Cain campaign worked to reassure supporters that the scandal would not end his bid. Steve Grubbs, Cain's Iowa chairman, said he was unaware of any volunteers who had backed away from their support.
He sent state backers a memo reminding them that Democrat Bill Clinton and Republican John McCain survived accusations of extramarital affairs during past races.
"What Herman Cain is going through is not unique," Grubbs told Reuters. "It's certainly a challenge and it's a distraction, but we're weathering the storm."
On the conference call with aides, Cain said the claims of an affair were taking an emotional toll on him and his wife but he promised to wage an upbeat campaign starting with an appearance in Michigan on Tuesday night.
Cain said his planned reassessment was similar to what he did at earlier key junctures in the race.
"During the summer we had to make some reassessments based upon our financial situation," he said. "We also did a reassessment after the Iowa straw poll and we made another reassessment after the Florida straw poll."
The charge of an affair differs from prior harassment claims in that White says she had a consensual long-term relationship.
White said the two met at a presentation Cain gave for the National Restaurant Association in Kentucky, and afterward the two had drinks. Cain invited her back to his hotel, she said, and the relationship took off.
The woman provided cellphone bills showing calls to a number she said was Cain's personal mobile phone. (Additional reporting by Kim Dixon; editing by Doina Chiacu and Mohammad Zargham)