Al-Maliki's dilemma has been deepened by the concerns of the country's most influential Shiite cleric, Grand Ayatollah Ali al-Sistani, who has indicated that the deal would be acceptable only if passed by a comfortable majority. The cleric is revered by Iraq's majority Shiites and he could sink the deal if he publicly speaks against it.

Iraq's government has two options if parliament doesn't pass the deal: renew the U.N. mandate governing the presence of U.S. troops in Iraq that expires Dec. 31, or ask them to leave immediately. Al-Maliki and top Cabinet ministers say neither option is good for Iraq.

Already, the ruling Shiite and Kurdish blocs appear ready to grant the Sunni Arabs and smaller groups their demand for a nationwide referendum on the pact to be held no later than July 30 in exchange for their support. If that happens, the deal could be approved by parliament, but torpedoed by a "no" vote in the referendum.

In Washington, Secretary of State Condoleezza Rice said the proposed referendum did not necessarily mean the pact will be put off. She said she had spoken to U.S. Ambassador Ryan Crocker and that her understanding is that the referendum process would not delay implementation of the security pact.

White House spokesman Tony Fratto said Washington remained hopeful that parliament would pass the agreement.

"They're going to keep working at it," he said. "It's a very good agreement. It's good for both Iraq and the United States. And so we'll keep an eye on what they're doing and hopefully they'll be able to get it across the goal line."

One of al-Maliki's deputies, Barham Saleh, said late Wednesday that parliamentary blocs were finalizing a political deal ahead of the vote on the security pact.

"We hope that tomorrow, the agreement will be presented to parliament as well as an agreement on a package of political reforms that the government and parliament will undertake," Saleh, a Kurd, told AP Television News late Wednesday.

If parliament approves the pact, it must be ratified by the Presidential Council, whose three members each have 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 Arab bloc, the Iraqi Accordance Front, has secured enough political gains in pre-vote dealmaking.

In addition to a referendum on the pact, the Front wants bigger representation for Sunni Arabs in the Shiite-dominated security forces and the release of thousands of detainees, mostly Sunnis, who are held in U.S.-run facilities without charges. 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.