In a prime-time address to the nation, Mr. Bush pushed back against the Democrats' calls to end the war. He said that "to step back now would force a collapse of the Iraqi government, tear that country apart and result in mass killings on an unimaginable scale."
The president said he was increasing U.S. troops by 21,500 to quell the country's near-anarchy.
Democrats were quick to respond.
"Escalation is not the change people called for in the last election," said Senate Majority Whip Dick Durbin, D-Ill., during a news conference immediately following Mr. Bush's speech. "In ordering more troops in Iraq, the president is ignoring the strong advice of most of his top generals."
"Twenty thousand American soldiers are too few to end this civil war in Iraq, and too many American lives to risk on top of those we've already lost" he said, adding that "If there's any surge that we need, it's a surge of diplomacy."
He added, "The Iraqis must understand that they alone can lead their nation to freedom. They alone must meet the challenges that lie ahead. And they must know that every time they call 911, we're not going to send 20,000 more American soldiers."
Even before Mr. Bush's address, the new Democratic leaders of Congress renewed their opposition to a buildup. "This is the third time we are going down this path. Two times this has not worked," House Speaker Nancy Pelosi, D-Calif., said after meeting with the president. "Why are they doing this now? That question remains."
Senate and House Democrats are arranging votes urging the president not to send more troops. While lacking the force of law, the symbolic votes will force every member of Congress to go on the record as either for or against the troop buildup — including some members who are running for president, reports CBS News correspondent Sharyl Attkisson.
Mr. Bush's plan drew criticism from Republicans as well as Democrats. "This is a dangerously wrongheaded strategy that will drive America deeper into an unwinnable swamp at a great cost," said Sen. Chuck Hagel, R-Neb., a Vietnam veteran and potential GOP presidential candidate.
Several Republican senators are candidates for backing the resolution against a troop increase. Sens. Susan Collins and Olympia Snowe of Maine, Gordon Smith of Oregon and Norm Coleman of Minnesota said they oppose sending more soldiers.
Republican Sens. George Voinovich of Ohio and John Warner of Virginia also might be persuaded. Warner said he supports the Iraq Study Group recommendations, which strongly cautioned against an increase in troops unless advocated by military commanders.
On Tuesday,, D-Mass., introduced legislation that would deny the president the money needed to send more troops unless Congress agreed first. It was unclear whether the bill would ever reach the full Senate, but it could serve as a rallying point for critics.
"The mission of our armed forces today in Iraq bears no resemblance whatever to the mission authorized by Congress," Kennedy said. "President Bush should not be permitted to escalate the war further and send an even larger number of our troops into harm's way, without a clear and specific new authorization from Congress."
The buildup runs counter to widespread anti-war passions among Americans and the advice of some — but not all — top generals.
It comes two months after elections that were widely seen as a call for the withdrawal of some or all U.S. forces from Iraq. Polling by AP-Ipsos in December found that only 27 percent of Americans approved of Mr. Bush's handling of Iraq, his lowest rating yet.
"He has really put it all on the line," says CBS News chief Washington correspondent Bob Schieffer. "The question you have to ask is, 'If this doesn't work, where will he go from there?'"