The referendum was a last-minute concession to Iraq's largest Sunni party, the Iraqi Islamic Party, which has long demanded that the agreement be put to a nationwide vote. "If there is no referendum, we will not vote yes to the agreement," said Omar Abdul Sattar, an Islamic Party lawmaker, speaking before parliament postponed the vote.
The security pact requires that U.S. troops pull out of cities and towns by summer 2009 and withdraw from Iraq by the end of 2011. But if Iraq's public rejects the security agreement in the referendum, scheduled for July next year, U.S. troops would have one year to leave, according to Shiite, Sunni and Kurdish lawmakers.
"We have given guarantees to conduct the referendum and will comply with the result, whatever that may be," said Haider al-Abadi, an influential lawmaker with Prime Minister Nouri al-Maliki's Dawa Party.
The vote on whether to approve the pact was originally scheduled for Wednesday.
U.S. forces are operating in Iraq under a U.N. mandate that expires at the end of this year. Unlike the mandate, the security pact calls for stricter oversight over U.S. troops and closer cooperation between Iraqi government and the U.S. military.
Iraq's Sunnis are concerned that the security agreement will allow the Shiite-led government to tighten its grip on Iraq. The Sunnis have made their support contingent on political changes that would bring about greater power-sharing among Iraq's sects. They also seek the release of thousands of Sunni detainees in U.S.-run prisons and greater Sunni representation in Iraq's mostly Shiite security forces.
On Wednesday, lawmakers said that two key Sunni demands -- the cancellation of a special criminal court that tries former members of Saddam Hussein's government and the reinstatement of members of Hussein's Baath Party to government jobs -- were still being negotiated. The continuing talks were the key reasons for the postponement of the vote, said Ali Adeeb, a Dawa lawmaker.
The Sunni bloc's 44 votes in the 275-member parliament are insufficient to defeat the security agreement; Shiite and Kurdish parties already claim the simple majority of 138 votes required to approve it. But in accommodating the Sunnis, the ruling coalition is bowing to the wishes of the country's preeminent Shiite spiritual leader, Grand Ayatollah Ali Sistani, who has said any deal should have the support of all of Iraq's parties in order to appear legitimate.
Abadi said that after parliament approves the agreement, as is widely expected, it would enact a law to hold the referendum. The Sunnis' political demands would also be voted on separately in parliament.
Mahmoud Othman, an independent Kurdish member of parliament, said lawmakers were still wrangling over the referendum. Iraqi politicians have often reversed their positions at the last minute, and it was possible they would do the same on Thursday, he said.
"The Kurds have no objection on this demand," said Othman. "But I don't think it's realistic to hold a vote in six months. Perhaps a year."
Other lawmakers questioned the legality of holding a referendum on the security agreement. "No article in the constitution allows that," said Muhsin Sadoun al-Karkary, a Kurdish lawmaker and a member of parliament's legal committee. "In case they put in this item of the referendum, then the government should solve this problem later because legally and constitutionally, such a referendum is not allowed."