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U.S. Marines hurriedly finished last-minute fortifications to protect voters in Iraq's referendum (search), but one tribal sheik in this Sunni Arab city wondered why they bothered: His tribesmen will vote as he tells them anyway. In fact, he offered to just vote on behalf of all 500 of them.

Tribes throughout this area are the critical, unknown factor in Saturday's landmark vote on a new constitution. They could tilt the electorate toward or against the draft charter that many in this Sunni Arab (search) area oppose. Few know the exact strength of tribes here in western Iraq or how they will urge their tribesmen to vote.

Some in the military believe the tribes have filled the void left after the fall of Saddam Hussein (search) and are capable of going to the polls in a synchronized vote.

"The decision to vote will be a tribally sanctioned decision," said Col. Stephen W. Davis, who commands Marine operations in western Anbar province. "The sheiks will decide what's good for their people."

Others contend that tribal influence had been waning throughout the country for years, noting that senior members of the Dulaimi tribe, the largest tribe in the province, and other tribes have fled to London or neighboring Jordan.

Some sheiks are nonetheless confident in their clout.

"All my tribe is going to vote," said Sheik Mohammed Abd al-Jaleel, who says he oversees 500 tribesmen in Haditha. "If you bring me a place for them, I will have all of them there."

Better yet, he suggested, "I can vote for all of them."

When the first drafts of the constitution were made public in August, al-Jaleel said he intended to order his tribesmen to reject the constitution. He changed his mind when some changes were made several weeks ago in the drawn-out negotiating process.

Parliament passed even more changes Wednesday in last-minute amendments aimed at overcoming Sunni Arab objections. Now Sunni Arabs have split, with some switching to a "yes" vote and others remaining deeply opposed. That could hurt Sunni opponents' campaign to defeat the charter, which depends on a heavy Sunni "no" vote to get a two-thirds rejection in three provinces. Anbar is one of the provinces opponents are counting on.

Whether all al-Jaleel's tribesmen will actually follow his call in the secret ballot is not known. But his idea of voting for them was not that unusual in the history of this region where tribes have historically been more powerful than Western-style governments. British colonial rulers in the early 20th century used sheiks to impose their will.

"If you were a political party, you would persuade tribal sheiks and landowners to join you, and they then marched their people to the polls or made up the results," said Roger Owen, a professor of Middle East history at Harvard University.

One resident, an engineer born and raised in Haditha, said there were about 10 tribes in the town, some with ties to the insurgency. His tribe lived in a neighborhood on the edge of the city and had so far been left alone by insurgents, he said, asking that his name be withheld for fear of militants' reprisals. He did not plan to vote Saturday.

Marine commanders said tribal disputes have periodically erupted in this area, part of a realignment of power. Insurgents, taking advantage of the absence of U.S. or Iraqi security forces, have challenged tribes for dominance and tribes were split on whether to support or oppose militants, U.S. commanders say.

"I think the tribes are very powerful, but they are afraid of the mujahedeen," said Habeeb Jundi, a Shiite Iraqi soldier in Haditha who had also fought in the nearby city of Hit.

Marines focusing on repairing Haditha's infrastructure have just started approaching local sheiks after U.S. troops led an offensive in this area last week, trying to both start reconstruction projects to build goodwill and gauge local opinion toward U.S. forces.

Most residents did not know where they could vote the night before the referendum — the U.S. military planned to send out Humvees with loudspeakers at the break of dawn on Saturday to announce the election center locations, said Capt. Shannon Neller of New York City.

"We want the people to find out fast, but we just don't want suicide car bombers to find out before them," said Neller, commander of Lima Company in 3rd Battalion, 1st Marine Regiment.