The proposed deal meets a longtime Iraqi demand for a clear timetable for the exit of about 150,000 American troops after years of war. But the Shiite-led government was struggling to meet the demands of political blocs, including a large group of Sunni Arab lawmakers, seeking concessions in return for supporting the agreement.

"There are complications, but we haven't lost hope yet," Foreign Minister Hoshyar Zebari told al-Arabiya television.

Lawmakers arrived at the parliament building for the planned vote in a session scheduled for Wednesday afternoon. But Parliament Speaker Mahmoud al-Mashhadani later said political leaders were working toward a settlement that will clear the way for a rescheduled vote Thursday morning.

"We have just been told that the general climate is definitely moving toward a solution," al-Mashhadani said. The leaders are in agreement over all issues except one and we have been told that they need time to settle it."

He did not say what the sticking point was.

Fearing insurgent attacks, the Iraqi army and police deployed additional forces around parliament.

Prime Minister Nouri Al-Maliki's ruling coalition appears to be assured of at least a slim majority in the 275-seat legislature. But the prime minister seeks a bigger win that transcends factionalism and sectarian divisions and reinforces the legitimacy of the pact, which could lead to full Iraqi sovereignty and close the bloody chapter that began with the U.S.-led invasion in 2003.

Shiite Lawmaker Ridha Jawad Taqi said the government's Shiite and Kurdish blocs, which account for about 140 seats, or a slight majority in parliament, were willing to hold a national referendum on the deal in 2009.

That is a concession to many Sunni Arab legislators, who have said they would support the security pact if it was put to a nationwide vote next year.

If that happens, the deal could be approved by parliament, but torpedoed by a "no" vote in the referendum.

A referendum would give the Iraqi people a chance to evaluate "whether their interests have been achieved," said Alaa Makki, a member of parliament's biggest Sunni Arab bloc, the 44-seat Iraqi Accordance Front.

A senior al-Maliki aide confirmed the concession by the Kurdish and Shiite blocs. Speaking on condition of anonymity because of the sensitivity of the subject, he said a draft bill containing provisions for the referendum and a package of political reforms demanded by the Sunnis would be voted on separately in parliament.

Under the deal, U.S. forces will withdraw from Iraqi cities by June 30 and the entire country by Jan. 1, 2012. Iraq will also have strict oversight over U.S. forces. The U.N. mandate that currently governs the conduct of American troops gives them freer rein, leading to Iraqi complaints that they are an occupying force intent on preserving U.S. interests in the Middle East.

In recent months, the U.S. military has increasingly been allowing Iraq's security forces to take the lead on operations such as routine security sweeps, but they continue to be the primary fighting force in joint operations.

The vote count will be as important as the overall result because the country's most influential Shiite cleric, Grand Ayatollah Ali al-Sistani, has indicated that the deal would be acceptable only if it passes by a wide margin. He could scuttle the deal if he speaks against it.

If parliament approves the pact, it must be ratified by the three members of the Presidential Council, each of whom has veto power. Two members support the deal. The third, Vice President Tariq al-Hashemi, is a Sunni Arab who could support it if he believes that parliament's biggest Sunni bloc, the Iraqi Accordance Front, has secured enough political gains in pre-vote dealmaking.

The Presidency Council, which includes President Jalal Talabani, a Kurd, and Adel Abdul-Mahdi, a Shiite, has led a flurry of contacts with political leaders over the past week to fashion a compromise that would push through the pact.

A lot of the negotiations barely relate to the security pact because political groups are seizing the opportunity to trade their support for concessions on other priorities.

In addition to the referendum, the Sunni Arabs and smaller groups in parliament have made their agreement to the pact conditional on a package of sweeping political reforms for a more equitable power-sharing formula between Shiite, Kurdish and Sunni communities.

Sunni Arabs want bigger representation in the Shiite-dominated security forces and the release of thousands of detainees, mostly Sunnis, held in U.S.-run facilities, but not charged with specific crimes. Under the security deal, detainees will be handed over to Iraqi authorities if arrest warrants are issued.

Al-Maliki says those demands should not be linked to the pact and has pledged to free detainees who were not involved in the insurgency.

Thirty lawmakers loyal to Shiite cleric Muqtada al-Sadr, who demands an immediate American withdrawal, oppose the pact. One of them, Zaynib Karim, complained that other political groups were flipping their position on the security deal out of expedience.

"Some blocs that were rejecting the security pact have acquired political gains, making them change their stance," she said.

Security was tightened throughout the capital ahead of the vote. But a roadside bomb killed two civilians Wednesday morning and wounded four in central Baghdad, Iraqi officials said.