KABUL — President Hamid Karzai’s challenger plans to call for a boycott of next weekend’s runoff election in an attempt to force the vote’s postponement until spring, his campaign manager said — a move that would dim U.S. hopes for a stable Afghan government for months.

Karzai rejected Abdullah Abdullah’s conditions for next Saturday’s vote, including removing top election officials whom the challenger accused of involvement in cheating in the first-round balloting in August.

Abdullah has called a press conference for 10 a.m. today to announce his final decision after Afghans and Westerners close to the challenger said he would withdraw. His campaign manager Satar Murad said the candidate might still change his mind, but that “as of now” he planned to call for a boycott.

A clouded electoral picture would further complicate the Obama administration’s efforts to decide whether to send more troops to Afghanistan.

The White House has been waiting for a new government in Kabul to announce a decision, but the war has intensified in the meantime. October was the deadliest month of the war for U.S. forces, with at least 57 American deaths.

Western officials hoped that Abdullah would make a gracious exit for the good of the country rather than denounce Karzai for fraud, a move that could sharpen tensions at a time the United States and its allies are seeking unity against the Taliban.

Secretary of State Hillary Rodham Clinton downplayed the prospect of an Abdullah withdrawal, saying it would not undermine the legitimacy of the election.

“I don’t think it has anything to do with the legitimacy of the election,” Clinton told reporters in Abu Dhabi. “It’s a personal choice which may or may not be made.”

Last-minute contacts were under way late Saturday between representatives of Karzai and Abdullah to resolve the crisis, and the challenger’s spokesman Fazel Sancharaki insisted no final decision on a withdrawal had been made. It appeared the uncertainty was aimed in part at allowing Abdullah to keep his options open until the last possible moment.

“I don’t know what will happen tomorrow morning,” Murad said. “We understand it shouldn’t go forward, and there should be an interim government immediately after the 7th or 8th of November” until a vote next spring, Murad said.

He said the approach of winter meant there was not enough time to organize an election that would be reasonably free of fraud.

“Therefore, it’s not good for the country, and it’s not good for the people,” Murad said. “We wouldn’t be having a legitimate government in the country if we went forward.”

If the election proceeds as scheduled, Abdullah will urge supporters to stay at home, and “our followers will not turn up to the election centers.”