KABUL — President Hamid Karzai was effectively handed a second five-year term today when his only challenger dropped out of the race, and the Obama administration said it was prepared to work with the man it has previously criticized to combat corruption and confront the Taliban insurgency.

President Barack Obama has been waiting for a new government in Kabul to announce whether he will send tens of thousands of new troops to Afghanistan. The war has intensified and October was the deadliest month of the eight-year war for U.S. forces.

Former Foreign Minister Abdullah Abdullah announced his decision to quit six days before the runoff election, after last-minute talks led by the U.S. and United Nations failed to produce a power-sharing agreement acceptable to Karzai, according to a Western diplomat.

In an emotional speech, Abdullah told supporters that he could not accept an runoff led by the same Karzai-appointed election commission that managed the fraud-marred vote in August. The runoff was set for next weekend after U.N.-backed auditors annulled nearly a third of Karzai’s votes as fakes.

“I will not participate in the Nov. 7 election,” Abdullah said, because a “transparent election is not possible.”

The Obama administration, which had been critical of Karzai’s leadership, appeared to accept the outcome.

Senior Obama adviser David Axelrod said most polls showed Abdullah would have lost the runoff anyway “so we are going to deal with the government that is there.”

“And obviously there are issues we need to discuss, such as reducing the high level of corruption,” Axelrod said on “Face the Nation.” “These are issues we’ll take up with President Karzai.”

Secretary of State Hillary Clinton congratulated Abdullah for a “dignified and constructive” campaign and said the United States “will support the next president and the people of Afghanistan, who seek and deserve a better future.”

Obama is still weeks away from deciding whether to send more troops. Top White House advisers said Obama’s painstaking review, ongoing since early September, would not be hampered by Abdullah’s withdrawal.

About 68,000 American troops already have been ordered to report to Afghanistan by the end of the year.

The top U.S. and NATO commander in Afghanistan, Army Gen. Stanley McChrystal, wants the Pentagon to send him an additional 40,000 troops to prevent the Taliban from letting al-Qaida once again use Afghanistan as a haven — as it was in the days leading up to the Sept. 11, 2001, terror attacks.

Despite misgivings over Karzai, the U.S. has little choice but to support a leader who was once the toast of Washington for his charm, his fluent English and his role as a conciliator in the wake of the Taliban collapse. Fluent in both major Afghan languages, he could reach out to different ethnic groups, including his fellow Pashtuns who also form the overwhelming majority of the Taliban.

But critics say he has been reluctant to rein in some of the former warlords whose support he sought to bolster his own political power but who are allegedly responsible for much of the corruption that plagues the government.

His own half-brother Ahmed Wali Karzai has been rumored to be involved in the drug trade, charges that he vigorously denies.

Karzai insists he fell out of favor in Washington when he openly criticized U.S. military tactics, including the heavy use of air power that has killed many civilians. McChrystal has ordered troops to use air power sparingly to avoid turning Afghans against the NATO mission.