Mir Hossein Mousavi, a former prime minister, has been away from Iran's political arena for two decades, but he now poses the greatest threat to the incumbent, Iranian President Mahmoud Ahmadinejad. Campaign rallies for Mr. Mousavi have drawn thousands in recent days, and in a new poll, 54 percent of those surveyed said they would vote for him. We take a look at the reformist candidate and the policies he promises to implement if elected.
Mir Hossein Mousavi has built his presidential campaign on criticism of the current president, Mahmoud Ahmadinejad.
He says Iran's government needs to treat the country's young people - the majority - with respect. He says the current government is not doing that.
"What is our mindset toward our own youth, our universities, our different generations? Should we consider them trustworthy?," he asked.
He also promises to end enforcement of the regime's strict Islamic moral codes by the religious police.
"Once president, I will immediately end the activities of the 'Moral Police Patrol,'" said Mousavi.
Mr. Mousavi's wife, Zahra Rahnavard, has played a key role in her husband's campaign and that is a first in Iran.
Since Mr. Mousavi tossed his hat in the ring, she has been at his side and has even campaigned alone, delivering speeches on his behalf.
Rahnavard focuses on women's rights, and some have dubbed her Iran's Michelle Obama. Her husband seems to have taken up the cause.
"Iranian women have lots of problems in their families and in Iranian society," said Mir Hossein Mousavi.
Mr. Mousavi has been active in Iranian politics for decades.
Just out of college, he joined a religious-nationalist political organization opposed to the Shah.
In 1979, the Islamic Revolution overthrew the Shah, who fled the country.
Soon afterwards, the Islamic Republic Party was founded. Mr. Moussavi edited the party newspaper until he was named foreign minister.
Then, when President Mohammad Ali Rajai was assassinated in a 1981 bomb attack, Ayatollah Ali Khamenei became President and Mr. Moussavi was named prime minister.
Mr. Moussavi held the post through the eight-year long Iran-Iraq war. He struggled to bolster the economy and combat inflation, nationalizing much of the private sector.
In 1988, Iran accepted a United Nations brokered ceasefire.
A year later, Ayatollah Khomeini died and Ayatollah Khamenei became Supreme Leader. The positions of President and Prime Minister were combined, with Ali Akbar Hashemi Rafsanjani elected president. Mr. Moussavi left politics.
Over the years, political activists have encouraged him to seek the presidency.
When former President Mohammad Khatami announced he would not run in this election, Mr. Moussavi said he would be a candidate. Now, he poses the primary challenge to President Ahmadinejad.