A boycott would severely undermine a vote intended to affirm the Afghan government's credibility. However, an Abdullah spokesman said no final decision had been made on the candidate's pullout, and that Abdullah will announce his decision Sunday morning. It was possible that word of the boycott was a negotiating tactic by the Abdullah camp.
The political stalemate in Kabul comes as President Barack Obama has been meeting with his advisers to try to determine U.S. strategy in Afghanistan, including troop levels. A weakened Afghan government will make it harder for Obama to get public support for his efforts.
Abdullah, who was once Karzai's foreign minister, put forward several conditions this week to avoid a repeat of the massive fraud of the August presidential election, including the replacement of the top election official and the suspension of several ministers.
He set Saturday as the deadline for his demands to be met.
A Westerner close to talks between the two sides said their agenda also included a power-sharing proposal by the challenger and cited both Karzai and Abdullah as saying that talks broke down Friday, prompting Abdullah to decide on a boycott of the Nov. 7 runoff.
An Afghan figure close to Abdullah said Saturday that the boycott decision came after a contentious and fruitless meeting Thursday over Abdullah's conditions for a runoff.
Both spoke on condition of anonymity, saying that the announcement must come from Abdullah himself.
The Afghan said a boycott was certain, and that Abdullah would likely tell his supporters to simply stay home during the vote.
The Afghan constitution says that any vote cast for a candidate who withdraws will not be counted. However, it does not specifically address a candidate who does not formally withdraw but urges supporters to boycott the polls.
A spokesman for the Afghan election commission said that it is too late for Abdullah to officially withdraw and that a boycott will not prevent the runoff from going forward.
"The election will be held and all procedures will go as normal," Noor Mohammad Noor said.
US Secretary of State Hillary Rodham Clinton said a runoff with only one candidate would not necessarily threaten the legitimacy of the process.
"We see that happen in our own country where, for whatever combination of reasons, one of the candidates decides not to go forward," Clinton told reporters in Abu Dhabi, United Arab Emirates. "I don't think it has anything to do with the legitimacy of the election. It's a personal choice which may or may not be made."
A spokesman for the Abdullah campaign, Fazel Sancharaki, said no decision had been made on a boycott and that the candidate would wait until the end of Saturday to see if his demands are met before making any announcement.
Abdullah canceled a planned trip to the Indian capital, Delhi, to attend a conference there Saturday. Sancharaki said Abdullah will announce his decision Sunday morning.
The runoff election in Afghanistan became necessary after widespread fraud in the first round of voting in August resulted in hundreds of thousands of Karzai's ballots being invalidated, pushing him below the required 50 percent margin to win. Concerns have been raised about a possible repetition of the ballot-box stuffing and distorted tallies in the second round.