US President George W Bush is preparing to unveil his long-awaited strategy for the future of US involvement in Iraq.
Any extra troops would be expected to be based mainly in Baghdad
The president is expected to bolster US forces in the country by sending more than 21,500 extra troops to Baghdad and the restive Anbar province.
But Democrats, who now control both houses of Congress, have made it clear they plan to oppose what they see as an "escalation" of the war in Iraq.
Speaker of the House Nancy Pelosi has pledged to hold a vote on any increase.
Officials say the president will announce the deployment of 17,500 additional troops in the capital, Baghdad - where Iraqi and US forces have launched a fresh offensive against militants.
Another 4,000 American troops will go to Anbar province.
The increases will take place in waves with the first additional brigade expected to arrive in Iraq within days.
"If we increase our support at this crucial moment, and help the Iraqis break the current cycle of violence, we can hasten the day our troops begin coming home," Mr Bush will say, according to excerpts of the address released in advance by the White House.
The US currently has 132,000 troops stationed in Iraq.
Mr Bush is also expected to recommend the transfer of responsibility for security to the Iraqis for all of the country's 18 provinces. Only three provinces are currently under Iraqi control.
But he will warn the Iraqi government of the need to fulfil its commitments.
"If the Iraqi government does not follow through on its promises, it will lose the support of the American people - and it will lose the support of the Iraqi people. Now is the time to act. The Prime Minister [Nouri Maliki] understands this," he will say in the address.
However analysts say the president will not tie the reinforcements to security benchmarks to be reached by the Iraqi government.
Mr Bush is also expected to announce an additional $1bn (770m euro, £0.5bn) in funds to strengthen Iraq's battered economy.
Veteran Democratic Party senator Edward Kennedy on Tuesday said sending more soldiers would be "an immense new mistake".
He suggested new legislation requiring congressional approval for any such move.
Senator Kennedy is a long-time opponent of the Iraq war
But the BBC's Justin Webb in Washington says many Democrats feel queasy about interfering in military matters - particularly if newly appointed US commanders in Iraq say they need the reinforcements.
The Democrats are likely to settle on symbolic votes of disapproval, our correspondent says.
However such a move would still be the biggest reconsideration of congressional support for the war since it began, he adds.
Mr Bush, whose address will be carried live on all US television networks, is due to speak in Washington at 2100 local time (0200 GMT Thursday).
In the UK, Prime Minister Tony Blair declined to say whether he would follow the US lead in increasing troop numbers in Iraq.
Britain has 7,200 troops in southern Iraq, mostly in the Basra area.