Taliban fighters warned Afghans not to take part in the war—wracked country’s upcoming presidential runoff, threatening on Saturday to launch a fresh wave of violence on polling day to stop them.

The warnings came on the first official day of campaigning for the Nov. 7 vote. The militant group denounced the race between President Hamid Karzai and challenger Abdullah Abdullah as “a failed, American process” and said its fighters would “launch operations against the enemy and stop people from taking part.”

The statement said Taliban militants will also cut off key roads and highways, and warned that anyone who casts a ballot “will bear responsibility for their actions.”

Taliban fighters killed dozens of civilians during the first round on Aug. 20, barraging several southern cities with rocket—fire and cutting off the ink—stained fingers of at least two people who cast ballots in the militant south.

Security fears are just one of the challenges election officials face as they scramble to organize a new election amid a swelling Taliban insurgency before the advent of winter, which begin around much of the country around the middle of November, isolating remote villages and cutting off roads with snow.

As campaigning began Saturday, several senior Abdullah campaign officials accused the top three members of Afghanistan’s Independent Election Commission of bias, saying they should be replaced to ensure the country’s upcoming runoff is fair.

A spokesman for the commission, Noor Mohammad Noor, denied the allegations and said it was “impossible” to replace them.

Under intense U.S. pressure, Mr. Karzai acknowledged last week that he fell short of the 50 percent threshold needed for victory in the August ballot after U.N.—backed auditors threw out nearly a third of his votes because of massive fraud.

The Afghan Independent Election Commission, dominated by Karzai supporters, is under huge pressure to avoid a repeat of the cheating, which discredited the government and threatened to undermine public support for the war in the United States, which provides the bulk of the 100,000 NATO—led force.

Abdullah officials singled out election commission chairman Azizullah Lodin, chief electoral officer Daoud Ali Najafi, and the commission’s deputy director, Zekria Barakzai. Abdullah’s running mate, Houmayoun Asafi, said the three were “openly working for Mr. Karzai.”

“If they are again in charge for the second round, the same thing will happen,” Mr. Asafi said, referring to the widespread fraud. “If the second round is also controversial, then the result will not be good.”

Mr. Noor said the officials had been appointed by constitutional procedures and cannot be replaced.

“It is impossible. Everything has been set up already according to the constitution, according to electoral law, and they will continue their work,” Mr. Noor told The Associated Press. “Mr. Karzai doesn’t have the right to replace them, and neither does Mr. Abdullah.”

The Obama administration is counting on a fair runoff to ensure the next government is legitimate. An outcome short of that is likely to raise further doubts about the wisdom of investing more U.S. troops and other resources in a counterinsurgency campaign in Afghanistan. A key pillar of that campaign is an Afghan government that is a credible partner of the U.S. and NATO.

Mr. Abdullah’s campaign spokesman, Fazel Sancharaki, said the three officials “should be replaced by people who are acceptable to both sides.”

He suggested Mr. Abdullah could boycott the vote if the officials were not replaced, but other officials in Mr. Abdullah’s campaign would not confirm the comment. The officials said the bias complaints had not been formally communicated to the election commission, but would be.

Mr. Sancharaki gave little evidence of alleged bias, saying only that the official tally the Independent Election Commission gave to Karzai - still below the 50 percent threshold - was slightly higher than the figures the U.N.—backed investigation indicated.

In an effort to tamp down cheating, Afghan authorities have said they will cut about 7,000 of the 24,000 polling stations that they set up for the August ballot. Some of those stations were in areas too dangerous to protect. Others never opened, enabling corrupt officials to stuff the ballot boxes with impunity.

Before the election commission announced final results last week, Mr. Lodin met repeatedly with Mr. Karzai. At the time, his group was challenging the findings of the auditors on the U.N.—backed Electoral Complaints Commission, a separate body. Mr. Lodin told The Associated Press that he saw nothing improper in those meetings and insisted he was not pressured to reject the auditors’ findings. Mr. Lodin told reporters that despite control measures, however, there was no way his commission could guarantee a fair vote on its own.

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