Sunni Arabs have reacted angrily to a decision by Iraq's Shia-dominated parliament making it harder to reject the new constitution in 12 days' time.
The constitution vote is seen as crucial to fulfilling US plans for Iraq
The two-thirds majority needed in three provinces to defeat the constitution will now be counted from all registered - as opposed to actual - voters.
Many registered voters may not show up because of violence, it is argued.
Saleh al-Mutlaq, of the Sunni group Iraqi National Dialogue, called the change a "clear forgery".
"They want this constitution to pass despite the will of the people," he added.
BBC Baghdad correspondent Caroline Hawley says violence, intimidation and US military offensives could keep many Sunnis away from polling centres.
The interim constitution drawn up under US administrator Paul Bremer in 2003 says the following about the issue:
"The general referendum will be successful and the draft constitution ratified if a majority of the voters in Iraq approve and if two-thirds of the voters in three or more governorates do not reject it."
Federalism, and forming of semi-autonomous regions
Terminology used to eradicate influence of former Baath regime
Structure of authority between presidency, parliament and government
But on Sunday, MPs said a No vote from two-thirds of "registered" voters was needed for a veto.
The new interpretation keeps the clause stipulating that only half of actual voters are needed for the text to be adopted.
Many Sunni Arabs oppose the draft constitution on the grounds that its federal provisions could lead to the break-up of Iraq.
In elections for the transitional parliament in January, less than 60% of registered Iraqis voted after Sunni leaders called for a boycott.
With growing Sunni-Shia tension ahead of the vote, analysts say the tactical alliance between Shia religious parties and the Kurds looks in danger of unravelling.
Kurdish officials accused Prime Minister Ibrahim Jaafari - a Shia - of seeking to monopolise power.
Violence in Sunni areas could deter some voters from taking party
They have also said he has failed to deal with the thorny issue of Kirkuk, the oil-rich northern city the Kurds see as their rightful capital.
However, President Jalal Talabani - a Kurd - played down his earlier calls for Mr Jaafari to resign.
"We don't think this is the time to change the government now," Mr Talabani said during a visit to Prague.
Mr Jaafari said that he had heard the president's comments, but did not have the time to respond to them.
"My time is taken up running the executive branch of the government. I will express myself forcefully when the time is right," he added.