In a letter to Bush released Monday, the Democrats backed a plan for the "phased redeployment" of troops.
"U.S. forces in Iraq should transition to a more limited mission focused on counterterrorism, training and logistical support of Iraqi security forces and force protection of U.S. personnel," the Democrats wrote.
Bush has consistently said there will be no such pullout until the fledgling Iraqi government can secure its position and Iraq's security forces can defend the country.
Republicans said the letter amounted to Democratic leadership surrendering to terrorists.
"The Democratic leadership continues to demand that American soldiers end their mission and wave the white flag of surrender to the terrorists that we are fighting in Iraq," Ron Bonjean, a spokesman for House Speaker Dennis Hastert, R-Ill., said in a statement.
Ken Mehlman, chairman of the Republican National Committee, issued a similar statement.
"Waving a white flag in Iraq may appeal to the net roots," Mehlman wrote, referring to Internet activists, "but it will embolden the enemy, encourage more terrorism and make America less secure."
Democrats had previously advocated reducing reduce troops levels in Iraq, but were split on the precise approach. During a recent floor debate in the Senate, Democratic Sens. John Kerry of Massachusetts and Russ Feingold of Wisconsin proposed legislation that would require troops to be out of Iraq by July 2007.
Senate Minority Leader Harry Reid, D-Nev., and other Democrats backed a measure that called for a phased redeployment to begin by Dec. 31, but did not set a deadline for all troops to be home.
The recent letter, dated July 30, is significant because signed by every top Democrat on committees with oversight of military, intelligence and international affairs it solidifies the Democrats' position and presents a unified front as members head into election season.
The letter also was signed by Reid and Rep. Nancy Pelosi of California, his House counterpart.
Pentagon leaders had hoped to begin withdrawing troops by the end of the year. But facing an uptick in sectarian violence around Baghdad, Defense Secretary Donald H. Rumsfeld last week extended the tours of 3,500 soldiers already in Iraq and announced plans to send as many as 5,000 additional troops into the Iraqi capital.
There are currently about 130,000 troops in Iraq. Boosting the troops by such a substantial number dashes Bush administration hopes of dropping the figure by tens of thousands by the fall congressional campaign.
Bush also has said that troop levels must be determined by military commanders on the ground there and that troops will remain until security conditions improve.