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A week before candidates can register for Afghanistan's presidential election, there is still no favourite to succeed Hamid Karzai, but deal making is going on in earnest behind the scenes.

Constitutionally barred from running for a third term, Karzai has been Afghanistan's only leader since the 2001 US-led invasion brought down the Taliban regime.

Twelve years later NATO troops are preparing to leave, a Taliban insurgency is as strong as ever, the country is polarised along ethnic lines and the government dependent on Western cash to survive.

The challenges facing Afghanistan's next president are immense and under Karzai's dominance, no obvious candidate capable of appealing to, let alone uniting disparate groups has emerged.

"For the first time in Afghanistan there is no favourite," said researcher Karim Pakzad from French think tank IRIS.

The last presidential election in 2009 was overshadowed by massive fraud but ultimately Afghans decided they were better off with the mercurial Pashtun leader who has proved adept at keeping any potential rivals at bay.

Despite huge concern about security and the possibility of an eventual return to civil war when NATO troops leave, Pakzad says: "There is no one capable of representing an alternative vision of peace, stability and progress."

But with Karzai out of the running for the April 5, 2014 vote, his potential successors have from September 16 to October 6 to register their candidacy.

In the absence of a favourite, the Afghan capital is awash with rumours.

"There is alliance building going on till the last minute," said Thomas Ruttig from the Afghanistan Analysts Network based in Kabul.

Several prominent members of the anti-Karzai opposition, based in the Uzbek and Tajik areas of the north have formed a new coalition, including Abdullah Abdullah who was Karzai's only rival in the 2009 election.

Other members are the former warlord turned governor of strategic province Balkh, Atta Mohammad Noor and the former communist fighter Abdul Rashid Dostum.

It is based loosely on the old Northern Alliance coalition that backed the US-led invasion that brought down the old enemies, the Taliban regime.

But while it has vowed to put forward a single candidate for the election, some analysts fear that it could collapse under the weight of its internal rivalries.

"It would represent a tremendous step forward of their political maturity if they indeed came up with a single candidate," said Ruttig, who thinks a three-way race is most likely.

He predicts former foreign minister Abdullah or someone else from the northern coalition will run against a Karzai loyalist and possibly a third candidate from another alliance still in the making, but warns "none of this is cast in iron yet".

On Tuesday, Abdullah gave a speech to hundreds of supporters in Kabul that had all the trappings of a campaign address.

"The government should not interfere in elections. If it does, it will have very bad consequences for Afghanistan," he said.

"I want the people to unify against the ongoing plot. We should all raise our voices and say we want elections," he added, referring to some calls that the polls should be postponed.

Many expect a Karzai loyalist to emerge victorious. Among his favourites are Foreign Minister Zalmai Rassoul and ex-ambassador to Pakistan, Omar Daudzai, recently promoted to interior minister.

The other unknown entity is the Taliban, which are still hugely influential in the south and the east.

In 2009 they carried out a series of attacks in a bid to torpedo the election.

The April vote will be a major test for the country's stability with most of the 87,000 foreign troops currently in Afghanistan to leave by the end of 2014.

Taliban leader Mullah Omar called the election a "waste of time" but has so far stopped short of threatening an increase in attacks.

"We'll boycott elections in April. We did not say we'll attack it, but the commanders on the ground will," one member of the Taliban told AFP. The militia dismisses Karzai and his allies as US puppets.

There has also been speculation about delaying the election or even that Karzai will try to find a way to cling on.

But for Moustafa Baaser, an analyst and a law student, that would be disastrous.

"Extending one person's mandate is not the solution, even having good tanks, an air force, more troops is not the solution," he said.

"We have to hold elections on time, and we have to have a new leader with new ideas and new energy."