The United States appears to be bracing for a rocky election in Afghanistan Thursday, a vote that could pose security challenges long after the polls close. 

While analysts say any of the top candidates in the presidential race could work hand-in-hand with the U.S. government, other concerns abound. 

Pre-election violence has already rocked the country in the days leading up to the election, most recently with a Taliban homicide bomber attacking a NATO convoy Tuesday outside Kabul, killing at least eight people and wounding more than 50. 

Such attacks, combined with Taliban threats to cut off the fingers of those who participate, have led to concerns that turnout could be low. Low turnout could give way to charges that the election results are illegitimate. 

Adding to that is concern about the kind of political post-election tumult that engulfed Iran, as Afghan President Hamid Karzai's top opponent Abdullah Abdullah suggests to supporters that if he loses it will be because of fraud. 

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To make the situation more unpredictable, even though Karzai is considered the front-runner, the winner must take a clear majority, leaving open the possibility that none of the candidates will garner enough support to avoid a runoff, thereby pushing the final vote off until October. 

The prospect of a prolonged period of indecision, coupled with concerns about the legitimacy of the election itself in a country where the authority of the central government is already in question, could pose a challenge to the United States and allied forces trying to escalate their offensive against insurgents while restoring political and economic stability. 

U.S. and NATO military commanders are planning for the possibility of unrest, but a stiff backlash could complicate the mission of improving the system of governance in the country. 

Secretary of State Hillary Clinton seemed to acknowledge the potential for trouble in a written statement Monday. She said the election "will not be without its challenges" and urged the country to behave responsibly. 

"We hope that, from top to bottom, every effort will be taken to make election day secure, to eliminate fraud, and to address any complaints fairly and quickly," she said. "Final results could take several weeks. We call on candidates and their supporters to behave responsibly before and after the elections." 

Clinton said the United States "remains impartial" in the election. 

Officials with NATO's election task force said Tuesday that insurgents will probably not be able to disrupt the election on a widespread scale. 

Australian Brig. Gen. Damian Cantwell, head of the election task force, said about 6,500 polling centers are expected to be open. 

Canadian Brig. Gen. Eric Tremblay, a spokesman with NATO's International Security Assistance Force, said judging by the number of attacks in recent days, insurgents would probably not be able to hit even 1 percent of those polling sites. 

But even if insurgent attacks are kept at bay during the election, the mere threat of attacks and reprisals could keep would-be voters in their homes. 

"There's an intimidation factor, no doubt," said Frederick Barton, with the Center for Strategic and International Studies. "There will be many, many polling stations that may not even open, probably hundreds." 

He called the threat of intimidation the greatest trouble-spot for Thursday. 

"The security situation has grown particularly troublesome," said Malou Innocent, foreign policy analyst at the Cato Institute. "It's rife with complications." 

She said low turnout, potentially due to Taliban threats, would lead to calls for a do-over election, though she doubts another election would be held. 

But that would only muddy the public perception of an election some Afghans are already skeptical toward. 

"Most Afghans believe that the next president has already been chosen by Washington, so the legitimacy of the election is already in question," Innocent said. 

While the presumed Washington pick is Karzai, Innocent said a win by Abdullah would not be a "setback" for the administration but a "boon," since Karzai has faced criticism in the West and his country for persistent corruption among officials and poor security even inside Kabul city limits. 

However, Abdullah, a former foreign minister, has contributed to a charged atmosphere ahead of the vote. His campaign manager was quoted last month as predicting street violence if Abdullah doesn't win. He said he was misquoted following an uproar over the comments, but now Abdullah uses similar rhetoric. 

"If there is no fraud in the election, it is clear for the nation who the winner will be and who the next president will be -- if they don't steal your votes," the candidate told thousands of supporters at a stadium in Kabul Monday. 

Ahmed Rashid, a fellow with the Pacific Council on International Policy, told the Council on Foreign Relations in an interview that all these factors could lead to a "constitutional deadlock" in the country after Thursday. From the Obama administration's perspective, he said the ongoing focus on the election would also "suck up the oxygen from development and from reconstruction."

Innocent and Barton, though, said there will probably not be widespread protests in the wake of the election, as were seen in the wake of Iran's disputed election in which Mahmoud Ahmadinejad was declared the winner. 

President Obama on Monday said the election will be an opportunity for Afghans to "choose the future that they want." 

He emphasized the stake the United States has in the security of the country, calling the Afghanistan fight a "war of necessity." 

The Associated Press contributed to this report.