KABUL - Opponents of Afghan President Hamid Karzai are scrambling to unite behind a single leader in a last-minute bid to field a unified team for the August 20 election, the brother of one of the main hopefuls said on Tuesday.

Karzai registered for the poll on Monday with a former opposition leader as one of his two vice presidents.

Other candidates have until Friday to sign up, but the opposition has so far struggled to come up with a ticket that might command broad enough support to effectively challenge him.

Abdul Salaam Jalali, aide and brother of one of the likely candidates, former Interior Minister Ahmad Jalali, said his brother and several rivals had met to try to form a united front.

Among those attending such discussions were former foreign minister Abdullah Abdullah, former finance ministers Ashraf Ghani and Anwar ul-Haq Ahady, and Karzai's current senior minister, Hedayat Amin Arsala.

They are working on a national agenda on issues like security, the economy, ways to find jobs, talks with opponents and the campaign against graft, Abdul Salaam Jalali said.

They are working on this agenda and if they agree, one person will lead the team, he said.

The prospective candidates were also holding talks with a tribal delegation which was pressing the hopefuls to form a common agenda and field a single team, he said.

The tribal delegation, composed mostly of influential figures from the mainly ethnic Pashtun south and east, would announce which candidate it would back after the talks, and most other candidates had agreed to accept its choice, he added.

Afghanistan has one president and two vice presidents, a system designed to encourage coalitions across ethnic lines.

Karzai, like most of his main rivals, is a member of the largest ethnic group, the Pashtuns. His vice presidential running mates are prominent members of the second- and third-largest groups, a Tajik and a Hazara.

Karzai's choice for his first vice president, Tajik former guerrilla chieftain Mohammad Qasim Fahim, attracted criticism from diplomats and rights groups because of accusations that linked Fahim to human rights abuses and corruption.

Karzai has been criticized in the West, including by U.S. officials, for tolerating corruption in his government. But U.S. criticism has grown more muted as officials in Washington have come to accept that he is likely to stay in office.

Karzai was in Washington on Tuesday, where he will have his first meeting with President Barack Obama since Obama's election last year, as part of three-way talks also attended by Pakistani President Asif Ali Zardari.