TEHRAN Mehdi Karoubi, the most liberal of President Mahmoud Ahmadinejad's three opponents in a June 12 election, is trying to expand his power base among reform-minded Iranians with a populist promise to dole out Iran's oil wealth.
Supporters of the former parliament speaker have plastered the Iranian capital Tehran with posters proclaiming Karoubi's dedication to fighting social injustice and poverty.
Karoubi has renewed his 2005 presidential campaign pledge to give shares of Iran's oil income to everyone aged over 18, but has not revealed the size of the handout or how it will be paid.
The idea parallels Ahmadinejad's vow to put some oil wealth on the table of each Iranian family -- a promise the president says he has helped to fulfill via the cash, loans and projects he has distributed in the provinces over the last four years.
Ahmadinejad also said this month that his government would issue "participation certificates," or Islamic bonds, to ordinary people, enabling them to invest in Iran's oil sector.
Karoubi, a moderate, mid-ranking cleric, has his own rural backers, but is likely to garner most of his votes from women, young people and intellectuals who fear damage to the economy and more curbs on social freedom if Ahmadinejad is re-elected.
An activist in the 1979 Islamic revolution, Karoubi switched to the reformist camp when President Mohammad Khatami was in power from 1997-2005. But some of those who had hoped for radical change blame him, as well as Khatami, for failing to stand up to Iran's hardline religious establishment.
Disenchantment with "reformist" leaders thwarted in that era may translate into a low turnout, compared with the mobilization that gave Khatami landslide victories in 1997 and 2001.
Karoubi, who favors economic liberalization and better ties with the West, accuses Ahmadinejad of isolating Iran with his vitriolic attacks on the United States, his combative line on Iran's nuclear policy and his belittling of the Holocaust.
"Our policy is one of detente. We want to have interaction with the whole world except Israel," he declared in March.
In Iran's clerical system, Supreme Leader Ayatollah Ali Khamenei, not the president, has the last say on national issues such as nuclear policy and relations with the United States.
Former Vice President Mohammad Ali Abtahi, a close Khatami ally, is among influential technocrats and politicians, from centrists to left-wingers, who have joined Karoubi's campaign.
"A rainbow coalition advises me on social, economic and political issues. This is my key to success," said Karoubi on a campaign trip to the central city of Isfahan in May.
His campaign manager is Gholamhossein Karbaschi, a dynamic former mayor of Tehran who launched big infrastructure projects but later faced corruption charges brought by hardliners.
Karoubi, born in 1937 in the western province of Lorestan, is the only candidate to form a party, the National Trust party.
He has faulted Revolutionary Guards officials for siding with Ahmadinejad despite a ban on armed forces' involvement in partisan politics. The Guards, and their 12-million strong Basij volunteer force, function in parallel with Iran's regular army.
"We are here to hold a free election without intervention of the Basij, the armed forces or rogue forces," Karoubi said after registering as a presidential contender on May 9.
Among the other challengers is Mohsen Rezaie, who commanded the elite Revolutionary Guards during the 1980-88 war with Iraq.
But analysts say the main race will be between Ahmadinejad and moderate former Prime Minister Mirhossein Mousavi -- Karoubi could come under pressure to pull out of the contest to avoid splitting the moderate vote and handing the incumbent victory.
Nevertheless, Iranian voters have sprung surprises before and Karoubi remains in with a chance, especially if he makes it into the run-off mandated if no candidate wins with more than half of all ballots cast, including blank votes.
(Editing by Alistair Lyon)