Abd al-Aziz al-Hakim, leader of Iraq's biggest Shi'ite political party, made the remark before thousands of cheering Shi'a in Al-Najaf on 11 August.
"Regarding the central-southern region, we think that it is necessary to form one entire region for central and southern Iraq due to the common characteristics of the residents of these parts and the same unjust policies which were adopted against them," al-Hakim said.
His comments come only days before the 15 August deadline for the Iraqi parliament to approve the new constitution.
Salih al-Mutlay, a member of the committee drafting the document, said there was no "time for such maneuvers."
Ayad al-Samaria, an official of the largest Sunni political, the Iraqi Islamic Party, said al-Hakim's remarks threatened an agreement because they opened up numerous side issues.
The role of federalism has been a major issue during negotiations on the constitution between Iraq's Shi'ite, Sunni, and Kurdish delegates. Iraq's Kurdish minority in the north already enjoy de facto autonomy.
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