Challenger Abdullah Abdullah has announced he's withdrawing from next Saturday's runoff presidential election in Afghanistan — a move that, as the Associated Press writes, raises "doubts about the credibility of the government at a time when the U.S. is anxious for an effective partner in the war against the Taliban."

Abdullah, who was foreign minister in the earlier years following the 2001 toppling of the Taliban, told his supporters today that he was dropping out because President Hamid Karzai refused to fire the top elections official who oversaw the August presidential voting that was marred by massive fraud.

According to the AP, Abdullah "stopped short of calling for an electoral boycott and urged his followers 'not to go to the streets, not to demonstrate.' "

Karzai got the most votes in that August election — and benefited most from those that were fraudulently cast. But the president did not reach the 50% necessary to avoid a runoff.

From Kabul, NPR's Soraya Sarhaddi Nelson says that Abdullah gave an emotional speech today, and that he said because the election process is not transparent enough he could not in good conscience continue.

In advance of Abdullah's announcement, The Washington Post wrote that his withdrawal "could make it more difficult for Obama to send additional U.S. combat troops to Afghanistan if the next government is not accepted by the Afghan electorate as a result."

Secretary of State Hillary Rodham Clinton said that Abdullah's withdrawal does not affect the special election's legitimacy, however, because Karzai gave it credibility when he agreed to take part.