Senior
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 Defense 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 defense 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 Democrats have promised a non-binding vote in both houses of Congress on the strategy.
'Too costly'
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
"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
Mr Gates said the "surge" would probably last "months, not 18 months or two years."
He added that the
Plan for
In his TV address, Mr Bush said fresh troops in
He said his country's commitment to
Mr Bush said the situation in
But the new troops, most of whom would be sent to
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.
Photo caption
US Defense Secretary Robert Gates