The Afghan presidential campaign narrowed from a chaotic field of challengers to a three-way race on Thursday, March 6 when the brother of the incumbent, Hamid Karzai, ended his own bid for power and threw his political weight behind the candidate believed to be the current president's favourite, The Guardian reported.

Qayoum Karzai's decision to back the low-profile former foreign minister Zalmai Rassoul creates a powerful ticket that appears to carry the covert blessing of the country's leader, and consolidates the strongest block of support from the last poll – ethnic Pashtuns from the south – behind one candidate.

Both men will stay on the ballot papers, as it is too late to make changes, but Qayoum Karzai will be telling his supporters to cast their vote for Rassoul to continue a decade of progress, he said at a brief press conference convened to announce the long-rumoured union.

Rassoul paid tribute to his new ally as a "great politician" who had made a brave move.

"Joining our team will benefit the nation of Afghanistan … we are sure that we will win the election," he told a gathering of journalists, campaign workers and influential elders.

They have two main rivals, who at the moment are both on track to win more than a quarter of the votes, recent polls show.

One is the academic and financial reformer Ashraf Ghani, who has allied with a civil-war era warlord to boost his chances after a dismal showing in 2009.

The other is Abdullah Abdullah, Karzai's main challenger in the last election, who has spent the intervening years building a grassroots support network in his northern base and beyond.

Even with a combined ticket Rassoul and Karzai lag far behind, but more aggressive campaigning in the south could rapidly change election dynamics and there is room for more high-profile support.

Several others from the 11-strong field of would-be presidents are in talks to join Rassoul's camp, campaign sources say, including a former defence minister and a grandson of the country's last king.

Most are Pashtuns from Karzai's Durrani tribe, which has ruled Afghanistan with just a few interruptions for over 200 years.

President Karzai had made little effort to hide his opposition to the idea of his brother taking his place in the presidential palace, and when a gathering of tribal elders reportedly backed Qayoum over his 75-year-old rival, he ordered them and the two candidates to reconsider.

Some of the group were worried that Rassoul would struggle to win votes in what should be his heartland. Although ethnically Pashtun, he has spent most of his life in Kabul or abroad, does not speak fluent Pashto, and in a breach of traditional norms that riles some conservatives, has never married or had children.

Although most of the prominent candidates are running on multi-ethnic tickets, with a modern, nationalistic platform, there is no doubt that strong tribal loyalties will play a key role bringing in voters on for the 5 April election.

Karzai's backing will bring name-recognition and votes in the family's home near Kandahar. But even if it means stiffer competition, his rival Abdullah said he welcomed the change because it meant a more "clear-cut" campaign.

"No candidate will be capable of making the collection of political leaders that President Karzai made last time around when he was the candidate, and almost all the known political leaders were in one camp … and I wasn't worried then," he said. "Things have changed. The younger generation are more educated and decide on a different basis."