US President George W Bush is expected to back a limited withdrawal of troops in an address on his Iraq war strategy.
The gradual pull-out would take troop numbers back to their level before Mr Bush ordered a build-up this year. The Democrats say more must be withdrawn.
Mr Bush's televised speech is expected to follow the advice of US commander in Iraq, Gen David Petraeus, who gave a progress report to Congress this week.
In his primetime televised address Mr Bush was expected to announce that he plans to reduce US troops by roughly 30,000 by next summer, if certain conditions are met.
At a White House briefing ahead of the speech, senior officials said the aim is to reduce the US force from 20 combat brigades to 15 by the middle of next year.
Senior Democrats have said the proposed reductions in US troop numbers are "insufficient" and do not represent a change in course.
Senator Jack Reed is expected to give the party's response shortly after Mr Bush's address.
House Democratic leader Nancy Pelosi has accused the president of effectively signing-off on an "open-ended" commitment that could keep US troops in Iraq for 10 years.