US Democratic leaders have urged President George W Bush not to increase troop levels in Iraq, days before he is expected to announce such a move.
Mr Bush may send thousands more troops to Baghdad
The Democrats, in control of Congress for the first time in 12 years, stepped up pressure ahead of Mr Bush's new policy initiative on Iraq.
The administration has confirmed it is replacing two top US military leaders in Iraq as part of the new strategy.
Mr Bush also announced a new national intelligence director.
The moves came ahead of the start of a neighbourhood-by-neighbourhood operation by Iraqi and US troops in Baghdad.
The operation, which is being launched over the weekend, is aimed at curbing the activities of Sunni insurgents and Shia death squads in the capital.
On Saturday, US troops killed four suspected bomb-makers and detained a fifth in an early-morning raid on a facility in an unspecified location.
One person was killed and six others were injured in a car bomb attack on a convoy carrying Baghdad police chief Maj-Gen Ali Yasser, police said. The police chief was unhurt.
On the first full day of Democratic control of Congress, the new leaders of both Houses called on Mr Bush not to raise troop levels in Iraq.
Correspondents say the president's policy initiative could include a deployment of 10,000 to 20,000 fresh troops in and around Baghdad to disarm militia groups there.
Nancy Pelosi and Harry Reid called for a phased withdrawal of forces
But in a letter to the president, Senate Democratic Leader, Harry Reid, and House of Representatives Speaker, Nancy Pelosi, said adding more combat troops would stretch the US military to breaking point with no strategic gain.
They instead urged a phased redeployment of US forces, starting in four to six months, with a re-emphasis on training, logistics and counter-terrorism operations in Iraq.
The BBC's Justin Webb in Washington says this is an aggressive move from the Democrats, setting the stage for a huge political battle.
Mr Bush cannot be prevented from sending more troops, our correspondent says, but he may pay a big political cost if the deployment is carried out amid fierce congressional opposition.
Defence Secretary Robert Gates on Friday confirmed earlier media reports that he was recommending changes at the top in the US military in Iraq.
Adm William Fallon will replace Gen John Abizaid as head of Central Command for Iraq and Afghanistan.
Lt Gen David Petraeus will take over from Gen George Casey as the leading ground commander in Iraq.
The other key personnel changes announced on Friday were the nomination of Director of National Intelligence John Negroponte to deputy secretary of state and of retired Navy Vice-Admiral Michael McConnell to Mr Negroponte's job.
US ambassador to Iraq Zalmay Khalilzad is expected to be put forward to replace John Bolton as US ambassador to the UN, sources say, with Ryan Crocker, US ambassador to Pakistan, replacing Mr Khalilzad.
Mr Bush says he needs to hold further consultations before he outlines his new strategy.
The expected policy shift follows a recent report from the Iraq Study Group, which said that US policy in Iraq was not working, and a Republican bruising in mid-term elections in November, blamed partly on the White House's handling of the conflict.