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Afghanistan is about to get a new leader for the first time since the Taliban were driven out in 2001 and replaced by the current president, Hamid Karzai.

Saturday's presidential runoff will be a historic event in Afghanistan, marking the first time in the country's long and often painful history that power has changed hands through the ballot box.

Karzai is barred from running again, and the only two names on the ballot will be Abdullah Abdullah, an ophthalmologist and former foreign minister, and Ashraf Ghani, a former finance minister and World Bank official.

The official campaign period has come to an end, and candidates are now observing a silence period as election and security officials make final preparations.

Abdullah enters the runoff as the front-runner, having won 45 percent of the vote in the first round on April 5. He's an ophthalmologist by training who became a medic with the mujahedeen, the Afghan fighters who battled the Soviet forces in the 1980s. Abdullah then became an adviser to the famous rebel commander Ahmad Shah Massoud.

Abdullah later served as foreign minister in Karzai's government, but split from Karzai in 2006 and challenged the incumbent president in the 2009 election. Abdullah received 30 percent of the vote but dropped out of the runoff against Karzai because he said the election was rigged in favor of the president.

Abdullah is half-Pashtun, Afghanistan's largest ethnic group; almost every ruler of Afghanistan since the 1700s has been Pashtun, including Karzai. Abdullah is also half Tajik, the second-largest group. However, because he made his name with Tajik-led rebel groups that fought the Soviets and the Taliban, he's viewed as Tajik by most Afghans.

Ghani, meanwhile, is seen as a technocrat, compared with Abdullah the diplomat and politician. Ghani, a Pashtun from the eastern province of Logar, won 31.5 percent of the vote in the first round. After completing high school in Afghanistan, he earned a bachelor's degree at the American University of Beirut. He later earned a doctorate in cultural anthropology at Columbia University, and taught at the University of California, Berkeley and Johns Hopkins.

He then became a senior official at the World Bank. After the fall of the Taliban, he returned to Afghanistan, playing a key role in the formation of the new government and serving as finance minister.

Over the years, Ghani has become a leading analyst on developing countries and is the author of the book Fixing Failed States.

Ghani also ran in the 2009 election, winning just 3 percent of the vote. Since then, he has largely reinvented himself by dressing in more traditional Afghan attire, using his tribal name, Ahmadzai, and otherwise trying to downplay the perception that he's more Western intellectual than Afghan. Ghani at one point held U.S. citizenship.

While the two men have different personalities and tend not to get along with each other, they share a desire to strengthen ties with the U.S. Both candidates have said they will sign a security agreement with the U.S. as soon he's elected.

The U.S. still has around 30,000 troops in Afghanistan, but is continuing to draw them down and will end combat operations by the end of the year.

President Obama said recently that he would like to keep about 10,000 troops in Afghanistan to help train and advise the Afghan military and to carry out counterterrorism operations. Those troops, who would be subject to the U.S.-Afghan security pact, will be gone by the end of 2016, Obama said.

Western officials have indicated that either Abdullah or Ghani would be preferable to Karzai, whose relations with the U.S. have badly frayed. He has refused to sign the security agreement

Abdullah and Ghani have few, if any, disagreements over policy issues, though Ghani has put forward a number of highly detailed policy proposals while Abdullah has stuck to generalities.

And Afghanistan is still a fledgling democracy where voters are more often swayed by tribal, ethnic and family affiliations than by who has the better tax plan.

The first round of the election was dominated by giant campaign rallies and televised debates — and periodic Taliban attacks on election offices. It was largely seen as a triumph of "new politics" that focused on media and voter engagement.

The second round, however, has been dominated more by the old-school politics of deal-making and closed-door meetings with potential backers. Abdullah has avoided debates and has held fewer rallies than Ghani, who's been campaigning more aggressively in an attempt to make up ground.

The two candidates spent much of this round trying to broaden their appeal by pulling in powerful supporters from Afghanistan's different ethnic groups. Of course, they did that from the outset by selecting running mates from different ethnic groups — Abdullah has a former Hazara warlord on his ticket, and Ghani a former Uzbek warlord on his.

While Afghanistan's former warlords have loyal followers and bring out many voters, many Afghans are dismayed by the fact that such figures will continue to play a major role in Afghan politics.

The current president has not endorsed either candidate, though Ghani has received the support of one of Karzai's brothers, Qayum. Abdullah has the support of another Karzai brother, Mahmoud.

Overall, Abdullah has an edge in the endorsement race, but that doesn't assure victory. It remains to be seen whether these endorsers can deliver votes. And fraud is still an X-factor; both candidates complained their vote counts were artificially low in the first round because of fraud.

While the first round was less violent than expected, the second round comes during peak fighting season and could be bloodier.

Western officials have expressed concerns that the results could be close, and Afghanistan's electoral bodies would likely have great difficulty adjudicating a tight result if there is significant fraud.

Abdullah and Ghani have both said that they will respect the outcome of the second round if the process is free and transparent. However, few expect it to be clean. And both candidates have warned that they can't control supporters who may not accept the outcome.

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