BAGHDAD, Iraq -The apparent acceptance of Iraq’s landmark constitution represents a major step in the attempt to establish a democratic government that could lead to the withdrawal of American troops.

The constitution seemed assured of passage yesterday after initial results showed minority Sunni Arabs had fallen short in an effort to veto it at the polls.

Opponents failed to secure the necessary two-thirds “no” vote in any three of Iraq’s 18 provinces, according to counts that local officials provided to the Associated Press. In the crucial central provinces with mixed ethnic and religious populations, enough Shiites and Kurds voted to stymie the Sunni bid to reject the constitution.

President Talabani of Iraq issued a decree setting December 15 for Iraqis to vote again, this time to elect a new parliament. If the constitution indeed passed, the first full-term parliament since Saddam Hussein’s fall in 2003 will install a new government by December 31. If the charter has failed, the parliament will be temporary, tasked with drawing up a new draft on which to vote.

But the outcome could further divide the nation, with many Sunnis fearing the new decentralized government will deprive them of their fair share in the country’s vast oil wealth. Large numbers of Sunnis voted “no,” and some of their leaders were already rejecting the apparent result.

While a strong Sunni turnout in Saturday’s referendum suggested a desire among many to participate in Iraq’s new political system, there were fears that anger at being ruled under a constitution they oppose could push some into supporting the Sunni-led insurgency.

“If the constitution was passed, the attacks will definitely rise against the occupation forces, and the security situation is going to be worse,” said Sheik Abdul-Salam al-Kubaisi, a prominent cleric with the influential Sunni Association of Muslim Scholars, which government officials accuse of links to the insurgency.

In a sign of the relentless danger, five American soldiers were killed Saturday by a bomb in Ramadi, a hotbed of militants west of Baghdad, the military announced. It was the deadliest attack on American troops since a September 29 bomb blast in the same town also killed five soldiers. A Marine was also killed by a bomb Saturday in the town of Saqlawiyah, the military said.

President Bush congratulated Iraqis on the referendum, which across the country saw few attacks and no deaths of voters in violence.

“The vote today in Iraq is in stark contrast to the attitude, the philosophy, and strategy of Al Qaeda, their terrorist friends and killers,” Mr. Bush said.