Iraqi Shiite tribal leaders are seen at a demonstration opposing a proposed U.S.-Iraq security pact Saturday in Najaf,100 miles south of Baghdad.

Baghdad ? The Iraqi parliament will vote Wednesday on a pact that would allow American troops to stay in Iraq for three more years, but the government’s hopes of winning a wide margin of approval to ensure the deal’s legitimacy appeared to be fading.

Prime Minister Nouri al-Maliki and his top ministers struggled Saturday to rally support for the pact, arguing that Iraqi security forces aren’t ready to stand on their own. A U.N. mandate for the American troop presence expires Dec. 31, and U.S. military operations would have to stop immediately without a new mandate or the legal cover of the pact being considered by parliament.

The defense minister warned that losing the protection of the U.S. Navy could even bring piracy to Persian Gulf waters like that preying on international shipping off the African nation of Somalia.

The vote originally was planned for Monday, but Parliament Speaker Mahmoud al-Mashhadani announced the new date after six hours of speeches by lawmakers closed out this week’s debate on the pact.

Al-Mashhadani said the vote could be held earlier than Wednesday if the Shiite-led governing coalition and other political groups reached an understanding, but that seemed unlikely after days of contentious debate and even some scuffling among legislators.

The speaker, a Sunni Arab, rated chances for the pact’s passage at “50-50.”

That assessment was a harsh one for al-Maliki, who needs a broad consensus. Failure to achieve that could deepen antagonism among Iraq’s political factions, which are heavily based on ethnic and sectarian loyalties.

The security pact emerged from nine months of tough talks between U.S. and Iraqi negotiators, and the Iraqi Cabinet approved it a week ago on the grounds that it provided a clear timetable for the withdrawal of U.S. forces after more than five years of war.

But many Iraqis see the American presence as a smear on national sovereignty, even if some believe it is needed for now to combat a lingering insurgency.

In the 275-seat parliament, the security pact also has become a flashpoint for attacks on al-Maliki in what could be a campaign warm-up ahead of provincial elections Jan. 31 and general elections late next year.

On Saturday, several lawmakers said it made no sense to approve a deal with a U.S. administration that has less than two months in office and argued a better option would be to negotiate a new pact once Barack Obama becomes president.

The proposed pact calls for American troops to pull out of Iraqi cities by next June 30 and to leave the country by Jan. 1, 2012. During his campaign, Obama said he would pull U.S. combat troops out of Iraq within 16 months of moving into the White House, which would be May 2010.

Iraq and the United States say the pact’s timetable for withdrawal could be sped up, but not delayed.

However, critics in parliament said they should have been given months, not days, to study and debate the deal. They said negotiations were conducted in secrecy without their input.

“What is really bothering me is that we are always in a hurry, and we later regret what we do,” said Maysoun Damlouji, a female lawmaker from a 25-seat secularist bloc led by Ayad Allawi, a former prime minister.

A lawmaker loyal to Shiite cleric Muqtada al-Sadr, whose militiamen have fought U.S. troops in major uprisings, complained that the document was first written in English and later translated into Arabic.

Even the pact’s supporters acknowledge it amounts to a compromise by Iraq because U.S.-led foreign troops would stay longer on Iraqi soil. But they say it’s a better alternative than extending the U.N. mandate, due to expire Dec. 31, that would allow American troops far more freedom to operate.

“The agreement has many negatives, but extending the mandate legitimizes the occupation and infringes on national sovereignty,” said Hadi al-Amri, a lawmaker from the Supreme Islamic Iraqi Council, the senior Shiite partner in al-Maliki’s coalition.