"I and my team, we consider ourselves as a key part of this new alliance and declare my support for Doctor Zalmai Rassoul," Qayum Karzai told a press conference in Kabul. (Agencies)
Former foreign minister Rassoul, a softly-spoken loyalist of Hamid Karzai, said: "From now on, both teams will fight for victory... and I'm sure with the blessing of God, and the support of the people, we will win."
President Karzai, who is constitutionally barred from standing again after serving two terms, has pledged not to publicly endorse any candidate in the April 5 elections.
But his brother's support of Rassoul is likely to be taken as a signal that Rassoul is the palace's choice to lead Afghanistan into a testing new era, when it will have to fight the Taliban insurgency without the aid of NATO combat troops.
The NATO combat mission ends in December, though a small number of US troops may stay in Afghanistan on training and counter-terrorism operations if a long-delayed security deal is signed with Washington.
Nearly 13 years after the fall of the Taliban, Afghanistan remains in a parlous condition, despite billions of dollars of military and development aid pouring into the country.
"I and my team, we consider ourselves as a key part of this new alliance and declare my support for Doctor Zalmai Rassoul," Qayum Karzai told a press conference in Kabul.