BAGHDAD, Iraq (CNN) -- Iraq's Sunni Arabs, who turned out in larger numbers for a constitutional referendum after boycotting January's parliamentary vote, are now flexing their political muscles for the December 15 assembly election, Sunni Arab officials told CNN.Three Sunni Arab groups -- the General Conference for the People of Iraq, the Iraqi Islamic Party and the Iraqi National Dialogue -- have formed a coalition, representatives of those groups said.Their announcement came a day after Iraqi officials announced that the constitution had won approval in the October 15 referendum, with 63 percent of voters casting ballots and more than 78 percent of them favoring the document. (Full story)In Washington, the U.S. ambassador to Iraq on Wednesday called the referendum a "landmark moment" for the Middle East.Zalmay Khalilzad helped the Shiite, Kurdish and Sunni lawmakers hammer out the document.He noted that Sunni Arabs "participated in substantial numbers" in contrast to January, when they stayed away from the polls during voting for the transitional assembly.Khalilzad said "positive political developments and continuing growth in the capabilities of the Iraqi forces" make is possible that the number of U.S. forces in Iraq could be reduced in 2006."I do believe it's possible that we could adjust our forces, downsizing them in the course of next year," he said.U.S.-led coalition forces in Iraq continue to battle a persistent insurgency believed to be dominated by Sunnis, who controlled the country until Saddam Hussein was overthrown in 2003.The U.S. military said Wednesday an American soldier died Tuesday evening in southern Iraq near Camp Bucca after a vehicle accident.The casualty raised the death toll for U.S. forces in Iraq to 2,001, the day after U.S. senators held a moment of silence in Washington to mark the 2,000th fatality. (Full story)Of the 2,001 deaths, 1,560 have been in hostile situations and 441 nonhostile, according to U.S. military figures counted by CNN.Sen. John Kerry called on President Bush Wednesday to begin reducing U.S. troop levels, with the objective of pulling out "the bulk of American combat forces by the end of next year."In a speech at Georgetown University in Washington, Kerry outlined a plan calling for the removal of 20,000 troops in the coming months, following the "completion of the democratic elections." (Full story)Anti-war activist Cindy Sheehan and 25 other protesters were arrested Wednesday as they lay on the sidewalk in front of the White House on the first night of what they said would be a weeklong vigil mourning U.S. deaths in Iraq.Sheehan, whose son died while serving in Iraq, was arrested last month in another White House protest. (Full story)Autonomous regions issueThe name of the new Sunni Arab coalition -- the Iraqi Accord Front -- has been submitted to Iraq's electoral commission for participation in December's election, an Iraqi Islamic Party spokesman said.A spokesman for the General Conference party, Sheikh Imad al-Deen Mohammed, said the coalition will "urge people to participate and vote during the coming general elections and to play a good role in the next government."The new assembly will be the permanent legislature. Its members, who will serve four-year terms, will choose the president, prime minister and Cabinet.Sunnis stayed away from the January 30 election, picking up few seats in the assembly. Over the year, many Sunnis decided to participate in the political process.Shiite and Kurdish leaders and Sunni officials spent weeks before the referendum devising compromises to make the constitution more palatable to Sunnis.Federalism -- the existence of autonomous regions -- remains one of the main issues of contention, Khalilzad said. Kurds in the north and Shiite Arabs in south favor language permitting such regions.Many Sunni Arabs, noting that Iraq's oil wealth is centered in those areas, say such regions would benefit those groups in the long run. Sunnis want a more centralized government.An autonomous Kurdish region already exists in the north, and some Shiites have expressed a desire for one in the south.One compromise allowed the constitution to be amended by the new assembly, a move that was agreeable to some Sunnis.Even though the constitution won by a wide margin, many of the minority Sunni Arabs voted against it.Khalilzad: 'Living document'Participation in the December 15 election for a parliament will benefit Sunnis, Khalilzad told the daily White House press briefing."More than 50 articles in the draft constitution require implementation laws to be passed in the next assembly," he said. "So being in the next assembly will also effect possible changes in addition to the normal amendment process."Khalilzad said "success" in the ethnically and religiously diverse Iraq depends "on a national compact" and he believes the "ratification of the constitution signals major progress toward that goal."He said the constitution is a working document, a fact that should assure opponents have "avenues for further changes" when the new parliament convenes."Like the U.S. Constitution, this is a living document that can be adjusted," he said.Other developmentsThe director-general of Iraq's Ministry of Culture, Nabeel Yasser al-Musawi, died in a drive-by shooting Wednesday while in his pickup truck in western Baghdad, Iraqi emergency police said.Khalilzad said the United States will work with Iraqis to "expand" security for lawyers and witnesses in the trial of Saddam Hussein. Iraqi lawyers and defense attorneys in the tribunal raised safety concerns after an attorney representing a Hussein co-defendant was killed last week. (Full story)A man the U.S. military describes as "an al Qaeda terrorist cell leader who personally assisted in at least three videotaped beheadings" was killed in a coalition raid on a "suspected safe house" in Mosul this weekend. The military, in a written statement, said Nashwan Mijhim Muslet, also known as Abu Tayir or Abu Zaid, and his assistant, Nahi Achmed Obeid Sultan, also known as Abu Hassan, were killed. "Several" insurgents were killed in raids on two "safe houses" Wednesday in the western town of Hit in Anbar province, the U.S. military said.