The vote in Iraq's parliament was scheduled to take place in Wednesday's session, but was put off until Thursday as the pact's supporters scrabbled to secure more votes among skeptical lawmakers.
Proponents of the agreement, which only reached parliament after weeks of arduous negotiations between Iraqi and U.S. officials, argue that the Americans are still needed to maintain recent security gains.
CBS News correspondent Elizabeth Palmer reports from Baghdad that the ruling Shiite and Kurdish parliamentary blocs have enough votes to approve the agreement, but the government wants it to win by a convincing margin - in part because one of this country's most influential Shiite clerics, Grand Ayatollah Ali al-Sistani, has said he can accept the agreement, provided it has broad public support.
Shiite Lawmaker Ridha Jawad Taqi told the Associated Press the government's Shiite and Kurdish blocs, which account for about 140 seats, or a slight majority in the legislature, were willing to hold a national referendum on the deal in 2009. That amounts to a concession to many Sunni Arab legislators, who have said they would support the security pact Wednesday if it was put to a nationwide vote next year.
So the deal, if approved in the parliamentary vote, could still be rescinded if it fails in the popular referendum.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 to the AP 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, also on Wednesday.
In a reminder of the still-tenuous security situation, Iraqi officials said Wednesday that at least two civilians were killed and four others wounded in a roadside bombing in central Baghdad, just hours ahead of the crucial vote.
Police and hospital officials said the blast targeted a minibus and that those killed were passengers.
The explosion happened in the Firdous Square area where thousands of followers of anti-U.S. cleric Muqtada al-Sadr rallied last week to protest the deal. The officials spoke on condition of anonymity because they weren't authorized to release the information.
America is also still paying a high price for involvement in the fight to secure Iraq. Two after a gunman in an Iraqi army uniform opened fire while they were distributing humanitarian aid in northern Iraq, the U.S. military said.
The security pact is meant to replace a United Nations mandate which grants coalition forces permission to remain in Iraq until the end of this year.
CBS News correspondent Charlie D'Agata reports that Prime Minister Nouri al Maliki's Cabinet overwhelmingly approved the deal last week, but the parliament gets the final say.
Only the Sunni lawmakers could stand in the way of the measure's passage, reports Palmer. They have been holding out for political concessions from the government, including an insistence that large number of prisoners be released. It was unclear Wednesday mourning whether any of the Sunni demands had been met.
More hard-line Sunni opponents, and some Shiites, including lawmakers allied with al-Sadr, have refused to adopt any policy which grants American soldiers an extended stay in the country, reports D'Agata.
If the security pact does not pass muster with politicians, officials in Washington and Baghdad worry there will be little time left in 2008 to hammer out details on an alternative.
Palmer reports that the U.S. would still have the option of going back to the United Nations and getting a extension of the Security Council mandate for Coalition forces. That would raise the possibility of a veto, but the Russians have indicated recently that they're unlikely to do that.
If that option fails, U.S. forces could be forced to return to their bases and cease all operations in Iraq. American officials have painted that as the worst case scenario, says Palmer, pointedly spelling out a wide range of activities they would no longer carry out, including providing security for Iraqi government officials.