President Barack Obama planned to declare today that the U.S. would end its costly combat mission in Iraq by late summer of 2010, but would delay a dramatic force reduction until after the country's national elections expected at the end of this year, senior administration officials said. The pre s ident was t o announce the Iraq policy this morning at Camp Lejeune in North Carolina, where thousands of Marines are soon heading to another war front, Afghanistan. The administration now considers Aug. 31, 2010, as the end date for Iraq war operations, said the officials. That timetable is three months slower than Obama had promised voters, but still hastens the U.S. exit. Even with the drawdown, a sizable residual U.S. force of 35,000 to 50,000 U.S. troops will stay in Iraq starting Sept. 1, 2010, under a new mission of training and advising Iraqi security forces; providing protection and support for U.S. and other civilians working on missions in Iraq, and targeted counterterrorism. There were no assurances that the residual force would not be pulled into battle should Sunni Muslim insurgent holdouts or disaffected Shiite Muslims resume wide-scale fighting. The potential size of that remaining force does not please leaders of Obama's own Democratic Party, who had envisioned a fuller withdrawal. Obama personally briefed House and Senate members of both parties about his intentions behind closed doors Thursday. But Republican Sen. John McCain, who lost the presidential election to Obama, offered his support for the plan today. "I think the plan is significantly different than the plan Obama had during the campaign," said McCain, referring to Obama's campaign pledge to pull combat troops out of Iraq within 16 months of taking office if possible. Defense Secretary Robert Gates and Adm. Michael Mullen, chairman of the Joint Chiefs of Staff, told lawmakers in the briefing that ground commanders in Iraq believe the plan poses only a moderate risk to security, McCain said. War critics are ready to hear Obama's public words. They see his much-anticipated announcement as the beginning of the end of a long, costly conflict. The last of the U.S. troops will be in Iraq no later than Dec. 31, 2011. That's the deadline set under an agreement the two countries sealed during George W. Bush's presidency. Obama has no plans to extend that date or pursue any permanent troop presence in Iraq. The existing U.S-Iraq agreement also calls for U.S. combat t roops to wi thdraw f rom Baghdad and other cities by the end of June. Administration officials spoke about Obama's Iraq decision under condition of anonymity to discuss details of the strategy ahead of the announcement. The Iraq war helped fuel Obama's presidential bid. Most Americans think the war was a mistake. More than 4,250 U.S. military members have died in the war in the seven and a half years since the United States invaded Iraq and toppled the regime of Saddam Hussein. Total Iraqi deaths are unknown but number in the tens of thousands and are perhaps above 100,000. From the Jan. 20 start of his presidency to his deadline for ending the combat mission, Obama has set t led on a 19-month withdrawal. He had promised the faster pace of 16 months during his campaign but also said he would confer with military commanders on a responsible exit. Officials said Thursday that the timetable Obama ultimately selected was the recommendation of all the key principals including Gates and Mullen. The timeline was settled on as the one that would best manage security risks without jeopardizing the gains of recent months. With 142,000 U.S. troops in Iraq, Obama plans to withdraw most of them; the total comes to roughly 92,000 to 107,000, based on administration projections. Officials said Obama would not set a more specific schedule, such as how many troops will exit per month because he wants to give his commanders in Iraq flexibility. "They'll either speed it up or slow it down, depending on what they need," said one official. Yet the officials made clear Obama wants to keep a strong security presence in Iraq through a series of elections in 2009, capped by national elections tentatively set for December. That important, final election date could slip into 2010, which is perhaps why Obama's timetable for withdrawing combat troops has slipped by a few months, too. One official said Gen. Ray Odierno, the top American commander in Baghdad, wants a "substantial force on the ground in Iraq to ensure that the elections come off." Another official said Odierno wanted flexibility around the elections. "The president found that very compelling," the official said. Obama has maintained that getting out of Iraq is in the security interest of the United States. He planned to emphasize in his comments today, however, that the U.S. has no plans to withdraw from its interests in the region and will intensify its diplomatic efforts. One of the officials said the plan that Obama will announce was recommended to him by Gates and Mullen on Wednesday and was driven by his Jan. 21 orders to draw up a schedule for ending the war. Obama made the final decision on Thursday, officials said. M c C a i n , a n A r i z o n a Republican, said his understanding is that the troops left behind would still go on combat patrols alongside Iraqis as part of the advisory role. "They'll still be in harm's way," he said. "There's no doubt about it." Obama had said all along he would keep a residual force in Iraq. "When they talk about 50,000, that's a little higher number than I had anticipated," Senate Majority Leader Harry Reid said before the briefing at the White House. Among others there was House Speaker Nancy Pelosi, who has also expressed concern about the troop levels. Some Republican lawmakers were skeptical for a different reason. They were concerned that troops might be pulled out too fast and security gains sacrificed. "While it may have sounded good during the campaign, I do think it's important that we listen to those commanders and our diplomats who are there to understand how fragile the situation is," said House Republican leader John Boehner. Violence is down significantly in Baghdad and most of Iraq, although many areas remain unstable. U.S. military deaths in Iraq plunged by two-thirds in 2008 from the previous year, a reflection of the improving security after a troop buildup in 2007. In the meantime, Obama has ordered the dispatch of 17,000 more American troops to Afghanistan, to fight resurgent Taliban insurgents. As U.S. troops leave Iraq, that would free even more forces for deployment in Afghanistan. ____ Associated Press writers Ben Feller and Steven R. Hurst contributed to this report.