Abdullah Abdullah, an ophthalmologist and former foreign affairs minister, is President Karzai's nearest competitor in Afghanistan's August 20 elections.
An ethnic Tajik-Pashtun, Mr. Abdullah, 48, is running as an independent candidate. He served in President Hamid Karzai's government for five years as foreign affairs minister. But he left in 2006 and has become increasingly critical of Mr. Karzai, even refusing a request from Karzai to be his running mate.
Abdullah's background includes years of resistance to Soviet and Taliban rule and a crucial role in the formation of the new democratic government after the United States ousted the Taliban in 2001.
He is best known for his close advisory relationship to Ahmad Shah Massoud, the legendary leader of the anti-Soviet Mujahideen who also fought the Taliban movement.
Abdullah campaigns as a populist carrying a message of change. He holds rallies outside the relative safety of the capital, Kabul, condemning "Kabul think" and the corruption he says is rampant in President Karzai's government.
His campaign platform includes the creation of a parliamentary system to dilute the power of the Afghan presidency. He is also calling for a system of electing officials for the country's 34 provinces and nearly 400 districts as opposed to the current system of appointments.