With Iran's conservatives divided, reformists are confident of a good showing by the moderate they support in Friday's presidential vote as they seek to avenge the disputed 2009 re-election of Mahmoud Ahmadinejad.
Thanks to a late surge of support, cleric Hassan Rowhani has emerged as a frontrunner with a real chance of forcing a run-off against the conservatives, analysts say.
The boost in his support came after the moderate and reformist camps joined ranks behind 64-year-old Rowhani after pressing the sole reformist candidate, Mohammad Reza Aref, to withdraw.
Sources said behind-the-scenes negotiations had taken place among the conservatives on Thursday in a last-ditch attempt to unify ranks and fend off Rowhani's challenge.
Unofficial polls show the conservative frontrunners to be former foreign minister Ali Akbar Velayati, Tehran mayor Mohammad Baqer Qalibaf and chief nuclear negotiator Saeed Jalili.
On Wednesday, efforts were reportedly made to persuade three of the five conservative candidates in the six-horse race to withdraw in favour of those shown to be leading the field -- Qalibaf and Jalili.
However, the conservative contenders dismissed any suggestion of withdrawing, ensuring that a total of six candidates will line up on Friday, reports said.
Rowhani's backers were busily working the phones and social media networks to urge a massive turnout in favour of the moderate-level cleric, who was Iran's chief nuclear negotiator from 2003 until 2005.
"I will vote for Rowhani, even though I do not know him at all and did not want to vote until yesterday," said a man named Ali on his Facebook page.
"I will vote because the consensus of those wanting to save Iran is on Rowhani."
Increasing support for Rowhani does not mean that reformists are united but it does boost his chances of forcing a run-off, according to Tehran-based conservative political analyst Mehdi Fazayeli.
Rowhani "is now regarded as one of the leading candidates and (his) chances of being in the second round, if there is one, are more evident," said Fazayeli.
If no clear winner emerges after Friday's vote, a second round will be held a week later.
The managing editor of the hardline Kayhan newspaper warned that the conservative candidates' stubbornness could "transform the river of conservative votes into a multitude of small streams".
Reza Marashi of the US-based National Iranian American Council advocacy group said it was vital for the conservatives to rally behind a single candidate.
"Unless the remaining conservative candidates coalesce, a splintered conservative field will face a candidate in Rowhani who has the backing of significant political and social forces in Iran," he told AFP.
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But Marashi also said Rowhani's chances would diminish "if large numbers of voters decide to stay home".
Rowhani has been endorsed by ex-presidents Akbar Hashemi Rafsanjani, a pragmatist, and the reformist Mohammad Khatami.
"I ask everyone, especially reformists" to vote for Rowhani, said Khatami, who was president between 1997 and 2005.
Rafsanjani, a pillar of the Islamic republic who was in power between 1989 and 1997, also backed Rowhani.
On Wednesday, he urged people to vote despite "doubts", adding, according to the Mehr news agency, that "surveys show that Mr Rowhani is ahead".
Four years ago, reformist hopes of returning to power were dashed by Ahmadinejad's re-election.
The two unsuccessful reformist candidates, Mir Hossein Mousavi and Mehdi Karroubi, immediately alleged fraud and called on their supporters to demonstrate.
Protests turned deadly and were crushed by the security forces. Mousavi and Karroubi have been under house arrest since 2011.
Some 50.5 million people are eligible to vote on Friday for a successor to Ahmadinejad, under whose presidency Iran has been isolated internationally over its controversial nuclear drive.
International sanctions slapped on Tehran in a bid to force it to give up its sensitive enrichment work have sparked a deep economic crisis, which has been the focus of the election campaign.
The United States on Thursday said it hopes talks on the nuclear issue will resume regardless of who wins the election.
Just eight male candidates out of nearly 700 registrants were approved by the hardline Guardians Council electoral watchdog to stand in the election. Two subsequently dropped out.
Those remaining are five conservatives mostly close to supreme leader Ayatollah Ali Khamenei: Qalibaf, Jalili, Velayati, former Revolutionary Guards commander Mohsen Rezai, and ex-communications minister Mohammad Gharazi.
Rowhani is the sole moderate candidate.
Khamenei, who has the final say in Iranian policy-making, has not made his choice public but has called for a massive turnout.
Ahmed Shaheed, UN special rapporteur on human rights in Iran, on Thursday called the election neither free nor fair, saying journalists and opposition leaders had been silenced ahead of the vote.