Iran's six presidential hopefuls were engaged in last-ditch campaigning Wednesday, hoping to woo the millions of Iranians still undecided between a slew of conservatives and a moderate cleric backed by reformists two days before the vote.

Momentum has built for cleric Hassan Rowhani, a former top nuclear negotiator, after the withdrawal of reformist Mohammad Reza Aref on Tuesday, and the endorsement of two ex-presidents, pro-reform Mohammad Khatami and pragmatist Akbar Hashemi Rafsanjani.

Rowhani, 64, faces stiff competition from the conservatives, in particular hardline top nuclear negotiator Saeed Jalili and Tehran mayor Mohammad Baqer Qalibaf.

According to an opinion poll published by the Mehr news agency, Qalibaf is leading the race with 17.8 percent support from the 10,000 voters canvassed, ahead of Rowhani with 14.6 percent. They were followed by Jalili with 9.8 percent.

The number of undecided voters stood at 30.5 percent -- about 15 million voters -- while 11.3 percent declined to answer.

The surge of support for Rowhani came after Aref, the only reformist candidate in the race, announced on Tuesday he was withdrawing at the urging of Khatami.

The former president subsequently asked voters "who seek the dignity and elevation of the nation," to vote for Rowhani.

Rafsanjani, a pillar of the 1979 Islamic revolution who himself was barred from running, added his backing for Rowhani, saying he was "more suitable" for Iran's highest elected office than the other candidates.

The closing of reformist ranks behind Rowhani prompted the conservative camp on Wednesday to urge their candidates to similarly coalesce.

"We now expect the conservative candidates to unite together without wasting time," wrote Hossein Shariatmadar, editor in chief of the hardline Kayhan daily.

"Backing one candidate will ensure victory, and the newly formed government can be comprised of the others," he said.

His calls were echoed by other conservative figures, including Habibollah Asgaroladi, who said, without elaborating, that consultations were under way to make "a coalition happen."

"To win, the conservatives must unite," said Asgaroladi, secretary general of a leading umbrella organisation for conservative groups.

Local media reported that pressure is mounting on ex-foreign minister Ali Akbar Velayati to withdraw so as to limit the division of the conservative vote.

Wednesday is officially the last day of the campaigning, but according to Iran's electoral law, candidates and their supporters have until 8:00 am (0330 GMT) on Thursday, 24 hours before polls open, to wind up their rallying.

This year's campaign has been low key compared to the previous vote in 2009 when boisterous rallies and street parties attracted crowds, as supporters of pro-reform candidates Mir Hossein Mousavi and Mehdi Karroubi faced off against the conservative-minded incumbent.

The rallies turned into heated street protests when Mahmoud Ahmadinejad was declared winner for a second term -- leading his opponents to allege massive voter fraud.

The months-long demonstrations turned deadly and were eventually crushed by the security forces. The two reformist leaders were later placed under house arrest.

Some 50.5 million voters are eligible to vote Friday for a successor to Ahmadinejad, under whose presidency Iran has been isolated internationally over its controversial nuclear drive.

Supreme leader Ayatollah Ali Khamenei, who has final say on all key state issues, including nuclear activities, has urged a high turnout on Friday.

"Every vote for any candidate is a vote for the Islamic republic; it is a vote to confidence in the establishment and the electoral process," Khamenei said last week.

Only eight male candidates out of nearly 700 registrants were approved by the hardline Guardians Council, Iran's electoral watchdog, to run in the race. Two have so far dropped their bids.