The Afghan president's older brother, Qayum Karzai, announced on Wednesday he was withdrawing from next month's presidential election and would back former foreign minister Zalmai Rassoul.
The alliance is the first public acknowledgement of weeks of negotiations aimed at consolidating the voting bloc of Pashtuns, the country's largest ethnic group, and brings together two candidates considered close to the current president.
"I will go to every mosque, every room and every guesthouse to get this team elected," Qayum Karzai told Reuters on the sidelines of a conference after announcing his withdrawal.
"We are hoping to win on the first round. This team represents the only hope for stability in this country."
Rassoul said negotiations on the election alliance had been taking place for two to three weeks.
NBC News Kabul Producer Fazul Rahim said many analysts believe Karzai pulled out as a result of pressure from his brother President Hamid Karzai.
The main opposition candidate, Abdullah Abdullah, said he was unconcerned about rivals joining forces. He narrowly lost the 2009 election to the incumbent Karzai.
While Abdullah is also a Pashtun, he is from a different group than the Karzai-Rassoul camp and represents the non-Pashtun north.
As a result, NBC’s Rahim believes the political campaign is taking an ethnic turn that could spark old rivalries and trigger violence if the results of the election are contested.
The polls are due to take place April 5. If they proceed normally, they will mark the first time in Afghanistan's history that power has been handed from one democratically elected government to another.
President Karzai is barred by the constitution from seeking a third term. He has not publicly offered his backing to any of the candidates.