A former ophthalmologist and resistance fighter against invading Soviet forces in the 1980s, Abdullah Abdullah served as Karzai's foreign minister from late 2001 to 2005.
Following the 2009 poll, which was widely discredited as massively fraudulent by the international community, Abdullah accused Karzai of "stealing" the vote.
Abdullah along with former World Bank economist Ashraf Ghani are seen as the two leading candidates in the race to succeed Karzai, with more than seven million people defying bad weather and Taliban threats to vote last week.
These suspicions have been fuelled, in part, by the construction of a lavish new house for him near the presidential palace and the nomination of his brother Qayyum as a candidate.
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Though Qayyum dropped out of the race, and his next anointed successor, Zalmai Rasoul, is believed to have fallen into third place behind the two leaders, Karzai is still seen as important to Abdullah's effort to secure the president's southern Pashtun base, the largest ethnic group.
Abdullah, who was born to an ethnic Pashtun father and a Tajik mother, is more associated with the northern Tajiks.
"The president of Afghanistan will have a role in the future. I'm not talking about an administrative role, I'm talking about a role in the politics of the country, a role at the national level, and he has served the country for the past 13 years.
"So that is something that is natural. What shape that role will take will depend on his decision", he said.
Asked whether he would be inclined to seek prosecution of Karzai or his brothers, who have been dogged by major corruption allegations for years, he added: "I would not go through that line that will be politics of vengeance and revenge and retribution. Our priority of the future government will be to deal with the priorities of the nation."