Ahmadinejad Camp

Ahmadinejad, who cannot seek a third mandate, secured an outright win with 62 percent in the four-man first round in 2009, provoking claims of fraud and the biggest protests since the revolution that deposed the Shah in 1979. If no candidate secures more than 50 percent, the runoff will be a week later.

Although Ahmadinejad, 56, is barred by law from running again, his rivals among fellow hardliners committed to resisting a dilution of the state's Islamist founding principles fear the confrontational incumbent will back a candidate - possibly his close aide Esfandiar Rahim Mashaei - to preserve his influence, even in the face of disapproval from the 73-year-old Khamenei.

"Unlike the first generation of revolutionary elite who played by the rules, Ahmadinejad's group has consistently pushed against regime red lines and at times even challenged the authority of the Supreme Leader," said Yasmin Alem, a US-based expert on Iran's electoral system.

"For them, ambition trumps allegiance to the regime's principles."

For his part, Khamenei is turning to a loyal alliance which includes his adviser Ali Akbar Velayati, another ally Gholam-Ali Haddad-Adel and charismatic Tehran mayor Mohammad Baqer Qalibaf, in the hope of securing a quick and painless election win, say analysts and diplomats.

But that informal coalition's final candidate has yet to be declared - a sign they are still sizing up the competition.