Several Sunni leaders drafting Iraq's constitution staunchly object to provisions included in the document and have found common cause with an unlikely ally: radical Shiites.
Sunnis and supporters of firebrand Shiite cleric Moqtada al-Sadr have said they will rally supporters to reject the constitution in October's national referendum.
Together the groups might convince two-thirds of voters in three provinces to vote down the document, prompting new elections for a national assembly that will draft another charter. A new vote would give both parties a chance to regain influence they lost when they boycotted last January's elections, leaving former exiled Shiite political parties and Kurds with a stronger hand.
While both groups have widely different visions for Iraq, both oppose federalism, which allows semiautonomous regions to spring up across the country.
"It's not the time for federalism under occupation. It will draw a lot of troubles," says Abbas Rubaie, the political director of the Sadr movement. This stance puts them at odds with the ruling Islamist Shiite parties like the Dawa Party and the Supreme Council of the Islamic Revolution in Iraq.
Sunni leaders who oppose the constitution say they will start a drive to inform Sunnis about what the document contains.
"We will explain everything in detail to them and we will leave it to their decision. We'll accept the result as long as we feel that no one has tampered with the vote," says Hussein Shukur al-Falluji, a Sunni member of the constitutional committee.
He says that in addition to federalism, Sunnis object to provisions regarding the dispersal of oil revenues, power sharing between the president and prime minister, and the description of the ethnic and religious identity of the country.
Efforts to include Sunnis during the drafting phase in order to quell the Sunni-led insurgency never really got off the ground. That galvanized Sunnis to call for a vote against the document even before the final draft was completed.
Adding to the strange bedfellows is the Association of Muslim Scholars, a group of extremist Sunni clerics with close ties to the insurgency who have been meeting with Sadr. Sunni clerics often dismiss Shiites as not true Muslims, while some Sadr leaders have preached against Sunni Wahhabis.
"When we have things in common with other people it's not necessarily an alliance," Mr. Rubaie says, noting that Sadr's followers support a provision keeping former members of the Baath Party out of government, the opposite stance of Sunnis. "Sayyid Moqtada has been visited by many groups and people, and that does not mean there is an agreement."
• Alan Enwia contributed to this report.