"Let me be clear: There is no military solution in Iraq and there never was," Obama said in excerpts of the speech provided to The Associated Press.
"The best way to protect our security and to pressure Iraq's leaders to resolve their civil war is to immediately begin to remove our combat troops. Not in six months or one year now," the Illinois senator says.
Obama's ardent opposition to the war has been a central theme of his campaign for the Democratic presidential nomination, and he has used it to distinguish himself from leading rival Sen. Hillary Rodham Clinton, D-N.Y. She voted to authorize the use of force in Iraq in 2002; Obama was not yet a senator.
Obama was trying to further sharpen that distinction Wednesday, spelling out his views on what the U.S. should do next.
He introduced legislation last January calling for withdrawal to start on May 1 and for all combat brigades to be pulled out by March 31, 2008. Anti-war Democrats and some Republicans want to bring all combat troops home in a matter of months.
Obama's push for withdrawal drew a sharp rebuke from Republican rival Mitt Romney.
"I think Barack Obama has disqualified himself for presidential leadership," Romney said in a telephone interview with The Associated Press. "If we take the kind of left turn represented by Barack Obama and his flee-in-the-face-of-success strategy, we'd be in a very different position as a nation."
In a letter to Bush on Wednesday, Clinton urged him to bring troops home faster and not to use his prime-time speech Thursday to declare new successes in Iraq. She said Bush's planned announcement of a reduction of 30,000 troops would have happened any way when the troops would have had to come home at the end of their 15-month deployment.
"He is in essence is going to tell the American people that one year from now the number of troops in Iraq will be the same as there were one year ago," she said after picking up the endorsement of the National Association of Letter Carriers. "Taking credit for this troop reduction is like taking credit for the sun coming up in the morning."
In criticizing the administration's current strategy, Clinton also linked the president's anticipated speech to the one he gave more than four years ago on an aircraft carrier under a banner that read "Mission accomplished."
"Mr. President, we don't need another mission accomplished moment," she said. "What we need is honesty and candor."
Obama's speech comes a day after Gen. David Petraeus, the top U.S. commander in Iraq, and U.S. Ambassador Ryan Crocker updated Congress on the situation in the war zone during two days of testimony on Capitol Hill.
Petraeus recommended that a 2,000-member Marine unit come home this month and not be replaced. That would be followed in mid-December by the departure of an Army brigade of 3,500 to 4,000 soldiers. An additional four combat brigades would be withdrawn by July 2008.
Obama said the U.S. and the Iraqi government should discuss how to go about withdrawing troops.
"We must get out strategically and carefully, removing troops from secure areas first and keeping troops in more volatile areas until later," Obama said in prepared remarks. Key excerpts were obtained by The Associated Press.
Although he stopped short of calling for an immediate pullout of all troops, Obama said there should be a clear and certain timetable.
"But our drawdown should proceed at a steady pace of one or two brigades each month," he said. "If we start now, all of our combat brigades should be out of Iraq by the end of next year."
By arguing that only combat brigades should be withdrawn there are 20 in Iraq, including five President Bush sent January Obama appeared to suggest that other U.S. troops could remain.
Underscoring the importance he was putting on the speech, Obama was being introduced by Zbigniew Brzezinski, who was national security adviser to President Carter from 1977 to 1981. Brzezinski has endorsed Obama's bid, and Wednesday's appearance would be his first on the candidate's behalf.
Obama rejected Petraeus' recommendation to maintain current troop levels through next summer to ensure security gains are maintained.
"The president would have us believe there are two choices: keep all of our troops in Iraq or abandon these Iraqis," Obama said. "I reject this choice."
Instead, he argued for creating an international working group of countries in the region and in Asia and Europe that would work to stabilize Iraq.
Democratic rival Chris Dodd criticized Obama and Clinton, contending that they were backtracking on "the need for a firm, enforceable deadline" on redeploying U.S. forces. Dodd said Obama "has a gift for soaring rhetoric, but, on this critical issue, we need to know the substance of his position with specificity."