More than 15 million Iraqis are eligible to vote in Saturday's "yes/no" referendum on the country's proposed new constitution.
Iraq's majority Shi'ites and Kurds are expected to vote to adopt the document, which includes provisions for a democratic, federal nation that guarantees the rights of women and minorities.
The Sunni Arab community, which was part of the ruling elite under ousted dictator Saddam Hussein, has expressed concern that its influence will be greatly diminished if the charter is adopted.
In a last-minute deal Tuesday, the main Sunni Arab political party - the Iraqi Islamic Party - announced it would support the draft constitution, after Shi'ite and Kurdish lawmakers agreed to a mechanism to consider amending the constitution if it is approved. But other Sunni groups have not publicly declared their support for the document.
A majority of all voters must approve the constitution for it to be adopted. It can be rejected if two thirds of voters in at least three of Iraq's 18 provinces vote "no."
If the charter is adopted, it will pave the way for new parliamentary elections by December 15.
If it fails, the national assembly will be dissolved, and a new transitional assembly and government will be elected in December and will begin the constitution drafting process again.