The candidate, Hasan Rowhani, ,moved to the forefront after another pro-reform candidate, Mohammad Reza Aref, pulled out yesterday. Conservatives were still fielding several competing candidates.
Rowhani gained key endorsements from two former presidents, both popular among reformers Akbar Hashemi Rafsanjani and Mohammad Khatami. Hasan Khomeini, grandson of Ayatollah Ruhollah Khomeini, founder of the Islamic republic, is also said to be backing Rowhani.
Official campaigning ends 24 hours before the polls open early Friday for voting to choose a successor to Mahmoud Ahmadinejad, who cannot run for a third term in office.
The West has imposed several rounds of punishing economic sanctions over Iran's suspect nuclear program, collapsing the currency and causing shortages and hardships.
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Four conservatives and a hard-liner remain in the race, opposing Rowhani. Aref's exit is seen as a reformist decision to unite, while multiple conservative candidates might split their side's vote.
If no one wins a majority in Friday's election, there would be a runoff with the top two finishers.
Tehran Mayor Mohammad Bagher Qalibaf, the apparent front-runner among the conservatives, joins former Foreign Minister Ali Akbar Velayati and former Guard chief Mohsen Rezaei in the race. Top nuclear negotiator Saeed Jalili is the most hard-line candidate in the race.
"To win the upcoming election, (conservatives) have to come forward with a single candidate. This is necessary," conservative politician Habibollah Asgarowladi told Iranian media today.