(Recasts with comments from another candidate, Karzai's office)
By Simon Denyer
KABUL, March 5 (Reuters) - Afghan President Hamid Karzai should stand down when his constitutional term ends in May and let an interim leader take over until an election in August, rival presidential candidates said on Thursday.
Abdullah Abdullah, who served as Karzai's foreign minister from late 2001 before being replaced in 2006, said the credibility of the poll was at stake, and insisted the country's political forces would be able to agree on an interim president.
"But the first thing is that he has to agree that that is necessary and then to find a solution," Abdullah said, referring to Karzai. "That will be a credit for him, that will be credit for Afghans as well."
Afghanistan has been plunged into a constitutional crisis over the election date. Karzai says the vote should be held according to the constitution, meaning it would have to take place in April, at least 30 days before his term ends on May 21.
But the election commission says that is not possible, partly because of the harsh Afghan winter, and almost everyone agrees. On Wednesday it insisted the election would be held on Aug. 20.
Many of Karzai's rivals worry the president is preparing to use the power of government to manipulate the poll in his favour, and want him to stand aside in May, as the constitution demands.
"If his intention is to stay there and do something just to get elected, then don't do that," Abdullah said.
A spokesman for Karzai said the president rejected the opposition's demand for an interim government, arguing this was not stipulated in the constitution.
"The president wants the stability and legitimacy of the system to be preserved, and will not take an action contrary to the constitution," said Siyamak Herawi.
TURMOIL, HORSE-TRADING AHEAD
Nevertheless, calls are growing for the speaker of the upper house of parliament and former president, Sibghatullah Mojadeddi, to take over if Karzai and his two vice presidents make way -- as he would if all of them died.
Mojadeddi, now in his 80s, served as president for just two months after the fall of the communist government in 1992.
"Karzai should step down and the president of the senate must become a caretaker governor, but he must not move into the palace," said "Prince" Abdul Ali Seraj, president of the National Coalition for Dialogue with Tribes of Afghanistan.
"Then people will see that there is change coming," said Seraj, the great-grandson of Abdul Rahman, the "Iron Amir" who ruled Afghanistan from 1880 to 1901.
Seraj is one of a handful of people who have already thrown their hats in the ring for the presidency, although no strong contender has yet emerged from the pack.
He predicted political turmoil if Karzai decides to stay on, against likely opposition from Afghanistan's parliament.
"If his people really want him for five more years, let him go back as an individual into the arena, and if people want him back they will vote for him," he said.
Karzai is very unlikely to give way without a fight, and it is now the horse-trading may really begin in earnest.
He could try to negotiate a way out of the mess, perhaps staying on with reduced powers or by making other concessions. Or he could brazen it out, even to the extent of declaring a state of emergency if parliament tried to impeach him, experts said.
Abdullah said he was being urged to stand as the candidate of the United National Front, a coalition of political parties and former warlords with its power base in northern Afghanistan.
But he insisted he had not yet decided whether he would run, arguing the post was "not that tempting" and had been discredited by Karzai, who he accused of driving the country into a "ditch".
Rising insecurity and rampant corruption have left Karzai increasingly unpopular, but with many deals still being cut behind the scenes between the country's political power brokers, a second term remains a possibility. (Editing by Valerie Lee)