Iranians cast their votes today amid widespread fears that the result could be rigged, as opponents of President Mahmoud Ahmadinejad believe happened four years ago.
Mr Ahmadinejad came from behind in 2005 to win 62 per cent of the vote in a second round, a victory popularly attributed to his support in the Basiji, a quasi-military volunteer group involved in administering elections.
"If there is no cheating, I am sure of [Mr Mousavi's victory in] the first round," said Saeed Laylaz, an analyst associated with the Mousavi campaign. Borrowing a football analogy, he said: "We started the campaign 3-0 down, now we are 4-3 up, so long as there is not a goal by the referee."
Mr Mousavi's supporters say it would only take a small amount of ballot-box tampering at each of Iran's 45,000 polling stations to swing the result. "Two million votes could change the elections," said Mohammed Atrianfar, a member of Mr Mousavi's campaign team.
Text messages have been circulating warning voters of potential traps at the polling stations: don't wear green, (the colour of the Mousavi campaign) and vote at schools, not mosques. In an indication of the paranoia among Mousavi supporters, some saw this advice as a trap in itself. Others told voters to bring their own pens because those at the polling stations could be filled with invisible ink.
There is no reliable polling data so it is unclear if Mr Ahmadinejad would need to interfere with votes to win. Mr Mousavi's camp is confident of a simple majority victory in the event of a 75 per cent turnout, obviating the need for a second round, but Mr Ahmadinejad has a strong populist appeal.
A victory for either side could create problems. "Iran has become more militarised recently and I am worried that if Mousavi's supporters feel he has lost because of cheating there will be clashes," Mr Laylaz said.
Sara, a Mousavi supporter, said: "I'm frightened about what will happen if Ahmadinejad loses. He is not a good loser."