Kabul ? Taliban militants threatened Afghans with violence Saturday if they vote in the Nov. 7 runoff presidential election, as President Hamid Karzai’s campaign ruled out any power-sharing deal to avoid another ballot.

Supporters of Karzai’s challenger, meanwhile, urged the country’s top three election officials to step down — alleging they were involved in rigging the first round of fraud-marred voting in August and should not be responsible for organizing the upcoming vote.

President Barack Obama’s administration is hoping the runoff will produce a legitimate government after massive ballot-rigging sullied the first-round vote Aug. 20. Another flawed election would cast doubt on the wisdom of sending tens of thousands more U.S. troops to support a weak government tainted by fraud.

The Taliban issued their warning on the first official day of campaigning for the runoff, denouncing the contest between Karzai and former Foreign Minister Abdullah Abdullah as “a failed, American process.”

They said fighters would “launch operations against the enemy and stop people from taking part” in the election, warning that anyone who casts a ballot “will bear responsibility for their actions.”

Taliban fighters killed dozens of people during the August balloting, firing rockets at several provincial cities and cutting off voters’ ink-stained fingers that indicated they had cast ballots.

The new threat adds pressure on U.S. and Afghan commanders to protect voters — one of the manifold challenges facing Afghanistan’s government and its international partners in organizing another election in less than three weeks. American commanders say the Afghans will bear the primary responsibility, with U.S. and NATO forces advising and standing by in case they’re needed.

The runoff was called last Tuesday after Karzai bowed to intense U.S. and international pressure and accepted the findings of a U.N.-backed panel that determined he fell short of the 50 percent threshold needed for an outright victory in the 36-candidate field.

Taliban threats kept thousands of people at home in August, helping push turnout below 40 percent. Officials fear even lower turnout this time, raising questions about the new president’s mandate no matter who wins.

“We’re sure that the elections will take place successfully, but because of the Taliban threats a minority of the people will go out and vote,” said Abdul Hadi, election chief in Helmand province, where Taliban influence is strong. “I ask the people to participate at the elections.”

Nevertheless, a Karzai spokesman said the runoff is necessary because there was no other lawful option for putting together a new government.

Some Obama administration officials had hoped Karzai and Abdullah could cut a power-sharing deal to avoid a costly and risky runoff, although such talk has faded in recent days.

U.S. officials hope that if Karzai wins as expected, he will bring Abdullah’s supporters and other opposition figures into the new government.