But Khaled al-Attiya, the deputy parliamentary speaker, said the government and the UIA were making a last-minute push to assemble a broader coalition.
"We do not want to pass this agreement with a difference of two or three or four votes," Attiya told the AFP news agency on the eve of the vote.
"For this reason there are continuing efforts to achieve a vast majority."
Attiya said the assembly would convene at 12:00 GMT instead of 10:00 GMT as originally planned, without providing further details.
Hoshyar Zebari, Iraq's foreign minister, told Al Jazeera on Wednesday that discussions between key political leaders were "ongoing to reach some consensus and the need to support and ratify the agreement by a clear majority".
"We do not want to pass this agreement with a difference of two or three votes. For this reason we are continuing efforts to achieve a vast majority"
Khaled al-Attiya, Iraqi deputy parliamentary speaker
He said it was a "crucial" day for Iraq and will show the world that the country is committed to its sovereignty.
A spokesman for the National Concord Front - the main Sunni bloc with 39 votes - said legislators were trying to hash out an agreement to meet the bloc's demand for political reforms related to national reconciliation.
"The reason for the delay is that the presidential committee of parliament and the presidential council have reached an agreement that includes a set of political reforms," Salim Abdullah told the AFP.
"Now there is a meeting to prepare a clear framework of the decision with the agreement of the political blocs."
The Sunni bloc had also demanded that the agreement be put to a national referendum next year, but Attiya said on Tuesday that proposal was dead in the water as far as the Americans were concerned.Withdrawal timeline
The agreement - the product of nearly a year of hard-nosed negotiations - was approved by Iraq's cabinet over a week ago with support from the major blocs representing the country's Shia, Sunni and Kurdish communities.
Iraq won a number of concessions in the deal, including a hard timeline for withdrawal, the right to search US military cargo and the right to try US soldiers for crimes committed while they are off their bases and off-duty.
The agreement also requires that US troops obtain Iraqi permission for all military operations, and that they hand over the files of all detainees in US custody to the Iraqi authorities, who will decide their fate.
The pact also forbids US troops from using Iraq as a launch-pad or transit point for attacking another country, which may reassure Syria and Iran.
But the accord has drawn fire from certain quarters, including followers of Moqtada al-Sadr, the Shia leader.