Three of Iraq's most powerful Shiite Muslim religious parties on Thursday formed a reshuffled alliance to field a slate in December's legislative elections, raising the prospect that balloting will once more break along ethnic and sectarian lines.

The agreement was reached after the Shiite alliance reportedly had been on the verge of splintering in recent days. The two parties that control Iraq's transitional government -- Dawa, led by Prime Minister Ibrahim Jafari, and the Supreme Council for the Islamic Revolution in Iraq -- will now work with political affiliates of Shiite cleric Moqtada Sadr. Sadr has a strong following among young, poor Shiites, but until recently he had rejected politics and referred to the last elections as illegitimate because of the U.S. occupation.

Vying against the Shiite coalition will be a second ticket, comprising the two main Kurdish parties, and a third bloc announced Wednesday by three Sunni Arab groups, which decided to participate in the elections after boycotting balloting for the transitional legislature last January.

U.S. officials in Baghdad and some Iraqi politicians had hoped the December elections would move beyond the divisive politics that stalled the formation of a transitional cabinet for more than three months earlier this year and deadlocked negotiations on the draft constitution that was endorsed by an Oct. 15 referendum.

"The hope was that they would begin to identify themselves as Iraqis with issues, rather than Iraqis with an ethnic background," said a Western official in Baghdad, who spoke on the condition he not be identified.

Wamidh Nadhmi, a political science professor at Baghdad University, said that "if the current landscape holds up, the election will be decided purely along sectarian lines and there will have been no progress."

Twenty-five members of a Shiite security force were killed Thursday in a Sunni-majority village southeast of Baghdad in a clash that illustrated the deep division between Shiite and Sunni Arabs. The security force had been sent to the village of Nahrawan to search for a kidnapped government employee, according to Capt. Ahmed Kadhum, an army intelligence officer.

The force had been drawn from the Interior Ministry's Karrar Brigade commando unit and Sadr's Mahdi Army militia, according to Kadhum and Mustafa Yacoubi, a spokesman for Sadr.

Elsewhere in Iraq, the U.S. military reported Thursday that three soldiers were killed Wednesday when roadside bombs struck their vehicles. One soldier died and four were injured in Balad, about 50 miles north of Baghdad, when a bomb exploded and small-arms fire erupted as their patrol passed, and two soldiers were killed when a bomb exploded near their convoy in eastern Baghdad.

Under Iraqi election law, voters endorse slates of parties, which vary across the country's 18 provinces, rather than backing individual candidates. Seats in the December elections will be awarded to groups in proportion to their share of the vote. Friday is the deadline for parties to register their groupings. The assemblage of political coalitions in recent days came after weeks of backroom negotiations and lobbying sessions by political elites at evening meals of rice and mutton that break the daily fast during the Muslim holy month of Ramadan.

Members of the Shiite majority have dominated Iraqi politics since January, controlling roughly 140 seats in the country's 275-seat transitional legislature. The addition of the Sadr supporters surprised many political observers because the cleric's militia had recently clashed in southern Iraq with members of the Badr Organization, a militia tied to the Supreme Council for the Islamic Revolution in Iraq.

"There is a lot of animosity between the two groups," Nadhmi said. "If it holds together, that will be a formidable grouping."

"We think the political process is a peaceful way to force the occupier out of Iraq, thus we decided to participate," said Abbas Rubaie, a spokesman for Sadr's political followers who said they had been promised 30 percent of the parliamentary seats earned by the alliance.

Politicians and analysts said Thursday that Ayad Allawi and Ahmed Chalabi, secular Iraqi politicians with longtime ties to the United States, may have a good showing in the December voting. The analysts said the men, who both have aspirations of being prime minister, could benefit from popular anger about the transitional government's inability to markedly improve security and basic services. Meanwhile, top Shiite religious leaders have decided not to endorse participation in the December voting.

Allawi, who served as interim prime minister last year, will join with Iraq's Communist Party and several prominent Sunni Arabs. And Chalabi, deputy prime minister and once a favorite of the Pentagon, is likely to leave the Shiite alliance to run with other political allies, possibly including the Hezbollah Iraq Party, said Haider Mousawi, Chalabi's spokesman.

Chalabi said he plans to focus on the economy and uniting the country across sectarian lines.

"Iraq does not need strongmen now," Chalabi said in a meeting with reporters in Baghdad this week, apparently referring to Allawi. "Iraq needs people who will talk to each other."

Despite some common ground, Chalabi and Allawi are bitter rivals.

Chalabi said he would travel to the United States next month, his first official trip since U.S. and Iraqi officials said in May that they were investigating him and allies on allegations of corruption and of passing secrets to the Iranian government.

Parties representing Iraq's Sunni minority will make their first move into elected politics after largely boycotting the January elections. On Wednesday they announced the formation of the Iraqi Concord Front, a slate made up of the Iraqi Islamic Party, the National Dialogue Council and the Iraqi People's Gathering.

The vast majority of Sunnis voted against the constitution in the Oct. 15 referendum, which passed with broad Shiite and Kurdish support. But Sunni political leaders said they were motivated to work toward the December elections by a provision added to the constitution days before the vote that will allow the next legislature to revise the document.

"This is the main reason why we want to participate," said Naseer Ani, head of the political office in the Iraqi Islamic Party. "If they give us a chance to change the constitution, we will take it."

Other Sunnis, led by hard-liner Saleh Mutlak, a prominent member of the Dialogue Council who has accused the government of fixing the referendum vote in favor of the constitution, plan to form their own alliance.

Correspondent John Ward Anderson in Baghdad and special correspondents Bassam Sebti in Baghdad, Hassan Shammari in Baqubah and Saad Sarhan in Najaf contributed to this report.

Deputy Prime Minister Ahmed Chalabi may leave Shiite coalition.