Senior US legislators from both parties have attacked President George W Bush's plans to send more than 20,000 extra troops to Iraq.
Defence Secretary Robert Gates is likely to face more stiff questions
Democratic Senator Joseph Biden called the move a "tragic mistake", while Republican Senator Chuck Hagel said Mr Bush had committed a dangerous blunder.
The announcement was part of Mr Bush's new strategy for tackling the conflict.
US Defence Secretary Robert Gates meanwhile will defend the plan before a hostile Congressional committee later.
On Thursday Mr Gates unveiled plans to boost troop numbers by 92,000 over five years, in an effort to increase the overall strength of the armed forces.
The defence secretary, who testified before a committee in the House of Representatives on Thursday, will appear before the Senate Armed Services Committee with the Chairman of the Joint Chiefs of Staff, Peter Pace.
The BBC's Justin Webb in Washington says the two men will be pressed hard on the wisdom of sending extra troops to Baghdad and Gen Pace is likely to be asked why he appears to have changed his mind about a policy he did not back in the past.
The Democrats have promised a non-binding vote in both houses of Congress on the strategy.
Mr Bush announced a "surge" force of 21,500 troops in a nationally televised speech on Wednesday night.
But alongside criticism from Democrats - who now control both houses of Congress - several Republicans publicly expressed their disappointment.
Loyalist Senator Norm Coleman said the cost in lives was "too great" to support Mr Bush's plan.
Another Republican Senator, George Voinovich, told US Secretary of State Condoleezza Rice at a committee hearing that he had finally lost faith in the president's Iraq policy.
"I've gone along with the president on this and I've bought into his dream and at this stage of the game I just don't think its going to happen," he said.
Sen Hagel said he thought the plan was a mistake.
"I think this speech given last night by this president represents the most dangerous foreign policy blunder in this country since Vietnam if it's carried out," he told the Senate Foreign Relations Committee.
Mr Gates said the "surge" would probably last "months, not 18 months or two years."
He added that the US could revise its plan if Iraqi leaders failed to keep to their commitments.
Plan for Baghdad
In his TV address, Mr Bush said fresh troops in Iraq would help to secure Baghdad's streets as part of the new strategy.
He said his country's commitment to Iraq was "not open-ended", and that he expected the government in Baghdad to fulfil its own promises.
Mr Bush said the situation in Iraq was unacceptable, and that responsibility for mistakes rested with him.
But the new troops, most of whom would be sent to Baghdad, would help secure neighbourhoods in the capital from insurgents, he said.
He said the effort would succeed where previous operations had failed, because this time troop levels would be sufficient to hold areas that had been cleared.