Iraq's Shia majority should be granted an autonomous federal state in the south of the country, a senior Shia leader has said.
Mr Hakim's brother, a revered ayatollah, was killed two years ago
Abdul Aziz al-Hakim made his call at a rally in the Shia holy city of Najaf, as Iraqi politicians debate the wording and balance of a new constitution.
Shia Muslims, who make up some 60% of Iraq's population, faced repression under Saddam Hussein's Sunni regime.
Iraq's Kurdish minority in the north already enjoy de facto autonomy.
The role of federalism and the balance of power between Iraq's Shia, Sunni and Kurdish communities have been key sticking points during negotiations.
Shia religious leaders have long backed calls for the creation of a federalised Shia south, but secular Shias have been cautious, fearing that it might grant excessive power to religious parties.
Secular Shias worry that Shia leaders want a religious federalism
Mr Hakim's Supreme Council for the Islamic Revolution in Iraq (Sciri) is a key player in Iraq's interim political scene.
Along with the Dawa party, Sciri makes up most of the Shia majority in Iraq's parliament.
"Regarding federalism, we think that it is necessary to form one entire region in the south," he told crowds in Najaf.
The proposal goes a step further than an earlier plan, already mooted during debates over the constitution, to merge a series of provinces around the southern city of Basra into a federal-style region.
That more cautious suggestion had already been opposed by Iraq's Sunni Arabs, who have consistently harboured concerns that Shia autonomy in the south, allied to Kurdish freedoms in the north, could leave them without vital revenues from Iraq's oil-rich regions.
Oilfields in the Shia south are vital to the regional economy
"We hoped this day would never come," a Sunni politician, Saleh al-Mutlak, told Reuters news agency,
"We believe that the Arabs, whether Sunni or Shia, are one," he said, adding a criticism of perceived attempts to divide Iraq along sectarian lines.
Laith Kubba, a spokesman for Iraq's secular Shia Prime Minister Ibrahim Jaafari, said the idea of a Shia region was "unacceptable".
In Najaf, however, Mr Hakim's call was met with wild applause and chants from the crowd.
Crowds had gathered on the anniversary of the killing of Mr Hakim's brother, Shia religious leader Ayatollah Mohammed Bakir al-Hakim, who died in a car bomb at Friday prayers in Najaf two years ago.