"There is ample classified information that shows the group remains a terrorist organization. De-listing them would signal that the US does not have a consistent policy towards terrorism,"
Despite the release from a Tehran prison of two jailed American hikers, there remain very few issues on which the US and Iran agree.
One is the decision to label the controversial Iranian exile group, Mujahideen-e Khalq (MEK), as a terrorist organisation.
Following a court order, US Secretary of State Hillary Clinton is now considering whether the group should be removed from the banned list.
Those backing the MEK are staging a very expensive campaign to call for the group to be removed - a move that would enable the MEK itself officially to lobby Congress.
The group's long list of detractors - and many Iran experts - warns against removing them from the terrorist list.
In a 2009 report, Rand, a non-profit Washington think-tank, called the group a "cult" and "skilled manipulators of public opinion".
Based inside Iraq, at a camp called Ashraf, north of Baghdad, the MEK has been on the US list of banned foreign terrorist organizations (FTO) since 1997.
The group carried out many attacks inside Iran after the 1979 revolution, and allied itself with Saddam Hussein's Iraq against Tehran's clerical rulers during the 1980s.
In recent months, though, a series of heavyweight political and military figures in the US have spoken out in favor of the MEK, calling explicitly for the group to be taken off the list of banned organizations.
They include high-profile former US government officials, politicians and retired military officers, often hired to speak for fees beginning at $20,000 a time.
The sources of funding for the pro-MEK campaign remain unclear, although paying former officials for public advocacy is commonplace in the US.
However, one US government official told the BBC that the MEK "trawls the halls of Congress" for support something he described as "highly unusual" for a banned organisation.
'No terror evidence'
MEK supporters operate through dozens of groups, some of which have placed costly full-page advertisements in The New York Times and Washington Post, and hired powerful Washington DC lobbying firms.
A spokesman for one firm, Akin Gump Strauss Hauer & Feld, denied that the company represented the MEK, but said it does lobby on behalf of a group called the Iranian-American Community of Northern California.
The spokesman described the group as "an independent US citizen's group that advocates for a democratic Iran."
But the group is the organizer of at least two events in support of the MEK and its website is focused on the de-listing campaign.
Ahmad Moein, a member of the group, recently told the Financial Times there was no justification for keeping the group on the banned list. He said the MEK was seeking a "democratic, secular, non-nuclear" Iran and "has halted all military activity since 2001".
Among those who have spoken out in favor of the MEK include former Attorney General Michael Mukasey, former UN ambassador John Bolton and former Homeland Security Secretary Tom Ridge.
"Start Quote Everyone is free to debate whether MEK should or should not come off the list, but as we speak they are still on the terrorist list"
End Quote Elliot Abrams Former White House adviser
Gen James Jones, President Obama's first National Security Adviser and former New York Mayor Rudy Guliani have also called for the MEK to be de-listed.
Howard Dean, a former Democratic presidential hopeful, has gone further, calling on the US government to recognise Maryam Rajavi, the leader of the group, as the legitimate president of Iran.
Speaking to the BBC, Mr Mukasey defended his position, saying there was "no evidence of [the MEK] being involved in any terrorist activity in the last 10 years".
Mr Bolton took a similar line, describing the decision to keep the MEK on the banned list as "a political act" and saying that taking payment for speaking was absolutely normal in the US.
"They should come off the list because when I was in the government, I saw no information that showed they are a terrorist organisation."
'Held against their will'
However, not everyone in Washington is as relaxed about the MEK's lobbying.
Elliot Abrams, an adviser to the White House under former President George W Bush, was also invited to speak at an MEK event, but chose not to attend.
"Everyone is free to debate whether MEK should or should not come off the list, but as we speak they are still on the terrorist list. So frankly, taking money from them to speak in support is worrying," he told the BBC
Reza Marashi, a former state department official, told the BBC he doubted that the group had any support within the US government.
"There is ample classified information that shows the group remains a terrorist organization. De-listing them would signal that the US does not have a consistent policy towards terrorism," he said.
The Iraqi government wants the group out of Iraq and has recently clashed with the inhabitants of the camp. MEK supporters say the US troop withdrawal from Iraq is leaving the group defenseless in a hostile country.
But many blame the leadership of the MEK for the predicament facing the residents of Camp Ashraf.
In 2005 a Human Rights Watch Report reported that 70% of Ashraf residents were held there against their will, and accused the MEK of torturing its own members.
Ali Safavi, a member of the political wing of the MEK, has admitted to the BBC that all the members in the camp have ended their marriages and are staying celibate.
But supporters of the MEK disagree entirely.
Ali Jafarzadeh, a key figure in the de-listing campaign, added: "Continuing the terrorist designation sends the signal that the outside world is prepared to preserve the regime."
Inside the US government, officials contend that the MEK does not have popular support and cannot bring democratic change to the country.
The European Union removed the MEK from its list of banned terrorist organizations in 2009.
Faced with a powerful lobbying force, state department officials will spend the coming weeks thinking about the ramifications of following in their footsteps.