Senator Carl Levin, a Michigan Democrat, who is expected to be chairman of the Senate armed services committee in the new Congress, said: "The first order of business is to change the direction of Iraq policy.
"We need to begin a phased redeployment of forces from Iraq in four to six months," he said.
Levin was speaking on ABC's television's This Week programme.
Levin said on the show that he hoped some Republicans would emerge to join Democrats and press the administration of George Bush, the American president, to tell the Iraqi government that a US presence was "not open-ended".
Bush has insisted that US troops would not leave Iraq until the Iraqis were able to take over security for their country.
Speaking on the same programme, Senator Joseph Biden, a Delaware Democrat who is expected to head the Senate foreign relations committee, said he supported Levin's proposal for a withdrawal.