Tensions have been building amid ever-broader hints from the Karzai camp that the president might reject the finding of the complaints commission.
As the dispute has dragged on, Karzai's aides have sought to capitalize on a vein of anti-foreign sentiment that is never far from the surface in Afghan politics. Three of the fraud panel's five members are foreigners appointed by the United Nations.
Last week, one of the two Afghan members, a Karzai ally, resigned, citing foreign interference.
A highly undiplomatic parallel dispute erupted last month when Peter Galbraith, the ranking U.S. official in the U.N. mission in Afghanistan, was fired after raising accusations that fraud allegations against Karzai's campaign were not being vigorously investigated.
The commission's finding was released in the form of raw data concerning the percentage of ballots invalidated in several categories. A number of independent election observers, including the U.S.-based group Democracy International, crunched the numbers and concluded that Karzai had fallen short.
Once the result is certified, a runoff -- if needed -- is to be held within two weeks. But Afghanistan's harsh winter is closing in, and mid-November is considered the cutoff for staging a new vote. After that, mountain passes are likely to be blocked by snow.
Clinton expressed confidence a runoff could be held despite the approaching winter.
"We have every assurance from Gen. McChrystal and the [NATO] command, as well as the Afghan security representative, that it is absolutely possible to do," she said.
Some closed-door discussions have centered on alternatives such as the creation of a coalition government. But Abdullah is seen as less likely to compromise in light of the panel's finding.
A "parade" of U.S., Afghan and international officials have been visiting the presidential palace in Kabul to counsel Karzai on his next move, said J. Alexander Thier of the U.S. Institute of Peace, who has taken part in the international monitoring effort.
He said he had gotten mixed signals on whether Karzai was likely to accept the conclusions of the panel. But he said that given the apparent scale of the fraud, "it would be untenable for Karzai to simply stonewall and not play ball at all."
Times staff writer Paul Richter in Washington contributed to this report.