KABUL, (Reuters) – Afghan presidential challenger  Abdullah Abdullah quit an election run-off yesterday after  accusing the government of not meeting his demand for a fair  vote, leaving doubts over the legitimacy of the next government.

A weakened Afghan government under President Hamid Karzai  would also be a blow for US President Barack Obama as he  decides whether to send up to 40,000 more US troops to fight a  resurgent Taliban in Afghanistan.

Karzai’s camp ruled out a coalition with Abdullah, dashing  hopes that might have been a way out of the impasse.

Daoud Ali Najafi, chief electoral officer of the  government-appointed Independent Election Commission (IEC), told  Reuters Afghanistan’s constitution meant the Nov. 7 run-off must  go ahead despite Abdullah’s decision.

A spokesman for U.N. mission chief Kai Eide voiced doubt  about the practicality of carrying on with the run-off vote.

“It’s difficult to see how there can be a run-off with only  one candidate,” said U.N. spokesman Aleem Siddique.

Abdullah’s withdrawal, and the IEC’s decision to push ahead  with the process, presents the possibility of foreign countries  now being asked to put more troops at risk to secure an election  in which the winner is already known.

Foreign troop deaths have hit record levels this year but  the NATO-led International Security Assistance Force in  Afghanistan would not comment on the dangers of such an  exercise.

“We continue to support the Afghan election process. We are  not a political entity, we are here to safeguard the election,”  ISAF spokesman Colonel William Shanks told Reuters.

Abdullah, an eye doctor and Karzai’s urbane former foreign  minister, appeared to rule out any immediate chance of a  power-sharing deal with Karzai in return for withdrawing, but  also told his supporters not to boycott the run-off.

His voice faltering and his eyes welling with tears,  Abdullah told hundreds of supporters, including white-bearded  tribal elders, in a giant tent used for grand assemblies that he  had reached the decision “in the interests of the nation”.

“As far as I’m concerned, the decision I have reached is not  to participate,” he later told reporters. “I have strong, strong  reservations about the credibility of the process.”

Karzai had been favourite to win the run-off after getting  more votes in an Aug. 20 first round marred by widespread fraud.  His campaign team also said the run-off would go ahead.

“Dr Abdullah’s decision has disappointed us,” Karzai said in  a statement, adding his team would accept whatever rulings are  made by the IEC and legal authorities such as the Supreme Court.

Afghanistan has been racked by weeks of political  uncertainty, with security also a major concern after the  Taliban vowed to disrupt the run-off. The Islamist militants  made a similar threat in August but failed to disrupt the vote  entirely.

The Taliban said Abdullah’s withdrawal made no difference.

“There will be no change of policy as far as we are  concerned,” Taliban spokesman Qari Mohammad Yousuf told Reuters  by telephone from an undisclosed location.

Some analysts were scathing in their assessment of what was  seen as a flawed election staged against the backdrop of  increasing violence after eight years of war.

“It is a shocking failure of efforts by the West and other  international communities to build a democracy in Afghanistan,”  said Norine MacDonald, president of policy research group The  International Council on Security and Development.

A strong and legitimate Afghan government is central to the  US strategy to quell rising Taliban violence. Obama had  already delayed the strategy decision on sending extra troops to  await the election result.