Voters in Iran go to the polls Friday to choose a president.  The race pits the incumbent, Mahmoud Ahmadinejad, against challenges from reformist and conservative candidates.  The campaign has been an unprecedented, no-holds-barred political fight.

Past presidential elections in Iran have usually been fairly staid. But not this time.

As journalist and Middle East expert Robin Wright points out, Iran's 2009 election campaign has been a surprisingly brutal affair. 

"Iran's two-week campaigns are always very intense," said Robin Wright. "And this campaign already features more diversity and open criticism than any previous election campaign."

The campaign has been marked by a level of personal attacks not previously seen in Iranian politics. For the first time, there have been live televised debates.

During one of the two-man debates, reformist candidate Mir Hossein Mousavi, considered by analysts to be the president's strongest challenger, directly blasted the incumbent for his economic policies and charging that he has tarnished the country's image internationally. 

President Ahmadinejad shot back, accusing Mousavi of getting backing from former president Ali Akbar Hashemi Rafsanjani, whom Ahmadinejad labeled corrupt.

Even more surprisingly, Mousavi's wife, Zahra Rahnavard, a political science professor, took a highly public role in the campaign, and was accused by the president of alleged academic violations.  Both she and Rafsanjani threatened to sue the president.

The atmosphere has become so poisonous that Rafsanjani issued a highly unusual public letter complaining of Supreme Leader Ali Khamenei's silence in the face of what Rafsanjani termed insults, lies, and false allegations.

There are two other challengers: former parliamentary speaker Mehdi Karroubi, who has the potential to split the reformist vote, and former Revolutionary Guard chief Mohsen Rezaei, who offers conservative voters an alternative to President Ahmadinejad. Both have also sharply criticized the president, but are considered long shots with little prospect of victory.

Robin Wright says the unprecedented level of personal attacks is because the focus is all on the incumbent.

"He is the biggest issue," said Wright. "All other candidates define their agendas in terms of his failures on the economy and the controversies over his foreign policy."

A recent poll by the New America Foundation, a private nonpartisan group, found President Ahmadinejad holding onto a lead, but with a large bloc of voters still uncommitted. 

Ken Ballen of the group Terror Free Tomorrow, which collaborated in the poll, says the results show that Iranian voters don't necessarily blame President Ahmadinejad for the poor economy.  But, he adds, they do want more freedoms, more engagement with the world, and more transparency from government.

"Even if they support President Ahmadinejad, it does not mean that they don't support these goals," said Ken Ballen. "So if he is re-elected, it's not on the mandate of the policies he has been carrying out heretofore. It's on a new mandate.  If he's listening to what the people want, whether they support him or not, it's change."

Flynt Leverett, director of the New America Foundation's Iran Project, says turnout will be crucial.

"It would seem as even with a relatively high turnout - 60 percent - Ahmadinejad is in a strong position," said Flynt Leverett. "If you get turnout higher than that, presumably that works to the benefit of his challengers, in particular Mr. Mousavi."

However, the closed nature of Iran makes truly accurate polling difficult, and no one can make a clear prediction.  Iranian elections are noted for surprises.  In 2005, the relatively unknown mayor of Tehran beat out other better known candidates and went on to final victory in the second round of voting.  The winner's name was Mahmoud Ahmadinejad.