James Bays, Al Jazeera's correspondent in Kabul, said the president, believed to be a frontrunner in the race, is facing a serious challenge from his main rival, Abdullah Abdullah.
"One of Hamid Karzai's aides told me a short time ago that he believed Dr Abdullah was the only main challenger," he said.
"But he said they still believed they were going to win at the first round at the election."
Abdullah, a former foreign minister, has drawn thousands of supporters to his election rallies around the country, blaming Karzai for Afghanistan's problems.
He is said to be counting on the votes of the country's youth, former foot soldiers from the Afghan civil war and women, with a campaign addressing female rights in the country.
Fears of violence
The presidential candidate is also hoping to attract the votes of the Tajik ethnic group, which his mother was from.
He has said the process of national reconciliation can start once the population supports the government, and when anti-corruption measures and increased development are created.
A wave of violence has struck during the lead-up to the election, with roadside bombs targeting policemen, Nato and US forces and civilians.
There are fears violence could threaten the country's second presidential election, with reports up to 700 polling stations may not open.