KABUL: Afghan presidential challenger Abdullah Abdullah quit an election runoff on Sunday 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 Taleban in Afghanistan.

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

Election officials said the Nov. 7 vote would go ahead with both names on the ballot but with Karzai the only candidate. “Based on election laws and based on the constitution there should be a second round. The constitution is clear,” Daoud Ali Najafi, chief electoral officer of the government-appointed Independent Election Commission (IEC), said.

But a spokesman for UN mission chief Kai Eide voiced doubt about the practicality of carrying on with the election. “It’s difficult to see how there can be a runoff with only one candidate,” said spokesman Aleem Siddique.

Abdullah, an eye surgeon 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 runoff.

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 favorite to win the runoff after getting more votes in an Aug. 20 first round marred by widespread fraud. His campaign team also said the runoff would go ahead.

“Dr. Abdullah’s decision has disappointed us,” Karzai said in a statement from the presidential palace, which added his team would accept whatever ruling the IEC made.

Asked later if a power-sharing deal with Abdullah was possible, Karzai’s campaign spokesman Wahid Omar said: “If it means a coalition government, certainly not.”