By Hamid Shalizi
KABUL, May 11 (Reuters) - Former foreign minister Abdullah Abdullah received a boost in the race for the Afghan presidency on Sunday when one of the pre-election favourites dropped out and backed his team ahead of next month's expected run-off.
Zalmay Rassoul, who finished third in April's first round with 11.5 percent of the vote, told journalists in Kabul he had endorsed Abdullah to strengthen national unity, and because the pair campaigned on similar platforms.
Preliminary results showed Abdullah and his closest rival, former World Bank economist Ashraf Ghani, sharing over 75 percent of the vote but neither winning an absolute majority.
The Afghan election commission will announce official first round results on Wednesday from the election, in which President Hamid Karzai was constitutionally barred from running again.
Evidence of widespread fraud reported by the country's Independent Election Complaints Commission have taken the gloss off the third presidential poll since U.S.-led forces drove the Taliban from power in 2001.
The vote marks the first democratic transfer of power in the country's history, however, uncertainty over the outcome risks stalling crucial foreign aid and economic reform.
Analysts said delays could also foment ethnic tensions and leave a political vacuum in which the Taliban could take advantage.
Rassoul, another ex-foreign minister, was considered one the pre-election favourites, having won the backing of some members of the powerful Karzai family. The presidential office said on Sunday that Hamid Karzai favoured no one candidate.
Rassoul's backing for Abdullah was seen as crucial, given most of his support is from the Pashtun dominated south of the country. Abdullah, who is half-Pashtun and half-Tajik, draws most of his support from the Tajik community in the north.
"Our platforms were similar to Rassoul, which is what brought us together," said Abdullah. "We want equality among the Afghan people and the next government will be monitored by the people."
Rassoul is the second of the eight candidates to quit the race and endorse Abdullah, after former Kandahar governor Gul Agha Sherzai made the same announcement last week.
Ghani, an ex-finance minister, has said he is not interested in deal making and his team will win a run-off on their own.
A second round is seen as a risky proposition in Afghanistan, given security concerns, the prospect of a low turnout and the cost - the bill for the first round was put at more than $100 million.
Underlining the security threat, Taliban insurgents announced their spring offensive would start on Monday. They vowed to target Afghan and foreign forces, and the political process.
Afghan troops are responsible for most security, as foreign combat troops prepare to leave the country by a year-end deadline.
Abdullah and Ghani say they will sign a bilateral security agreement permitting a small force of American troops in Afghanistan for counter-insurgency and training purposes.
Karzai had refused to sign the pact. (Editing by Jeremy Laurence and Sophie Hares)