At a news conference in Kabul, Qayum Karzai, the current president's older brother, backed former Afghan Foreign Minister Zalmai Rassoul in Afghanistan's April presidential election, the Wall Street Journal reported.
Mahmood Karzai, another of the president's brothers, said the merger of the two campaigns came after weeks of intense negotiation.
"The constituency of both candidates is practically the same," he said. "Both teams are paying close attention to the Afghan traditions, to tribal traditions. So we wanted to make sure that our constituents are in line with our thinking."
Despite his withdrawal, Qayum Karzai's name will remain on the ballot along with the 10 other candidates, officials said.
"There will be no change in the ticket," said Hamid Saboory, a senior aide to Qayum Karzai. "What we are trying to do is convince everybody who are our supporters on voting day to not vote for Qayum, but vote for Rassoul."
The election is scheduled to take place April 5. President Karzai is barred by the constitution from seeking re-election.
He has not officially endorsed a candidate in the race.