The Supreme Council for the Islamic Revolution in Iraq (Sciri), is one of the two main Iraqi Shia parties.
Ayatollah Hakim was one of Iraq's best-known Shia figures
It has remained committed to the US-led political process in Iraq, although it has criticised US officials for not giving Iraqi leaders more authority.
Sciri's former leader, Ayatollah Muhammad Baqr al-Hakim, was among about 100 people killed in a massive car bombing in the Shia holy city of Najaf in August 2003.
Ayatollah Hakim opposed Saddam Hussein from exile in Tehran before returning in May 2003 to widespread acclaim from thousands of Iraqi Shias.
Before his death, the ayatollah offered qualified support to the US-appointed Governing Council - giving the body much-needed legitimacy among Iraqi Shias.
He had many supporters in Iraq, but his close connections to the ruling clerics in Iran made the US - and some Iraqis - wary of him.
But analysts say that as his stance shifted towards the US, his backing in Iran became weaker among Iranian conservatives but stronger among reformists.
Ayatollah Hakim's brother, Abdel Aziz, has taken over as leader of Sciri. He has carried out negotiations with the US and represented Sciri at international meetings.
He held Sciri's only seat on the US-appointed Iraqi Governing Council but has not been included in the interim government.
Senior Sciri official Adel Abdel-Mahdi, however, has been appointed finance minister in the new cabinet.
Sciri's armed wing - the 10,000 strong Badr Brigade - was renamed the Badr Organisation after coalition officials banned party militias in September 2003.
Despite the name change, the organisation, which was funded by Iran for many years, is still charged with "maintaining security and stability" according to Abdel Aziz al-Hakim.