The Office to Consolidate Unity, a group often seen as the vanguard of the battered reformist movement, had told its members that voting would only endorse a system in which candidates would be vetted by a strict Islamic watchdog.
But it made a sharp U-turn on Monday after Tehran Mayor Mahmoud Ahmadinejad, linked to the hardline Revolutionary Guard and Basij Religious Paramilitaries, made it through to a second round run-off with Rafsanjani, scheduled for Friday.
"Pro-reform students have decided to back Rafsanjani to prevent the establishment of a totalitarian system if Ahmadinejad wins the vote," student leader Sajjad Ghoroghi said.
"We have formed a campaign office, called The Students' Anti-Fascism Headquarters, whose members will travel to villages to encourage people to vote for Rafsanjani," he added.
In the first round last week, Rafsanjani won 21% of the vote while Ahmadinejad came in second with 19.5%.
Students and other liberals have little love for Rafsanjani whom they associate with some dark chapters of Iranian history.
Mahmoud Ahmadinejad says the
reformers are poor losers
Rafsanjani's 1989-1997 presidency was marked by modest economic and social liberalisation as well as attempts at bridge-building with the United States.
Many of Rafsanjani's detractors are now marshalling their forces behind his presidency bid, fearing ultra-conservative Ahmadinejad has been using influence in military and paramilitary groups to get votes.
Ahmadinejad's allies deny this and say reformists are just bad losers. Ahmadinejad's campaign has appealed to the religious poor.