A day after Bush's proposal was pelted with bipartisan criticism on Capitol Hill, Sen. John McCain , R-Ariz., a leading presidential contender for 2008, said he supports the plan. He tried to shift the burden to war critics.

McCain said those advocating the start of a troop withdrawal, which includes many Democrats, "have a responsibility to tell us what they believe are the consequences of withdrawal in Iraq. If we walk away from Iraq, we'll be back, possibly in the context of a wider war in the world's most volatile region."

In a second day of hearings, there was still plenty of congressional skepticism about Bush's strategy, which would add 21,500 U.S. troops to the 132,000 already in Iraq.

Rep. John Murtha, a Pennsylvania Democrat who oversees military funding, said he will propose tying congressional approval of war funds to shutting the Guantanamo Bay military prison in Cuba. Other conditions he said he is considering include not extending troop deployments and giving soldiers and Marines more time to train between deployments.

Bush is expected to ask in February for $100 billion for the wars in Iraq and Afghanistan. His plan to add troops is estimated to cost an additional $5.6 billion.

Bush struck a defiant note in an interview to be televised Sunday by CBS on "60 Minutes." Asked if he believes he has the authority to send additional troops to Iraq no matter what Congress wants to do, Bush said: "I think I've got ¡ª in this situation, I do, yeah. And I fully understand they will ... they could try to stop me from doing it, but, uh, I've made my decision and we're going forward."

Democratic leaders in the House and Senate intend to hold symbolic votes in coming days to demonstrate the extent of opposition to Bush's troop increase. House Speaker Nancy Pelosi, D-Calif., told fellow Democrats in a closed-door meeting she intends to allow the Senate ¡ª where several Republicans have been vocal in their criticism of the president ¡ª to begin debate first.

In a sign the war was increasing anxiety among House Republicans, GOP leaders arranged an informal listening session Friday in a Capitol meeting room to allow members to voice their apprehensions about the war.

"Members from diverse parts of the country, diverse regions have questions they want answered," said Rep. Adam Putnam of Florida, a member of the Republican leadership. "There is a healthy skepticism about whether we're fundamentally changing things on the ground."

Bush invited top Republican leaders from the House and the Senate to Camp David for at least part of the holiday weekend.

Bush's new plan was devised after the Nov. 7 elections, in which voters handed congressional control to the Democrats in large part because of the war. An AP-Ipsos poll this week found approval for Bush's handling of Iraq hovering near a record low ¡ª 29 percent of Americans approve and 68 percent disapprove.

Bush on Friday sought support for his new Iraq military buildup in telephone calls to Jordan's King Abdullah II and Egyptian President Hosni Mubarak. Secretary of State Condoleezza Rice is traveling to the Middle East this weekend to sell Bush's plan and try prodding regional peace efforts.

Testifying before the Senate Armed Services Committee on Friday, Defense Secretary Robert Gates and Gen. Peter Pace, chairman of the Joint Chiefs of Staff, said that the effectiveness of the increase in troops depends heavily on the Iraqis and that Bush has made it clear to them additional U.S. support is not open-ended.

"I think we'll see fairly quickly whether they are prepared to step up to the plate and perform as they have promised," Gates said.

If the operation is successful, "we in fact may be able to begin drawing down some of our troops later this year," Gates added. "But that will depend entirely on the situation in the ground."

Gates and Pace also assured lawmakers there are no immediate plans to attack targets in Iran. In his speech this week, Bush vowed to disrupt Iran's aid to insurgents in Iraq and "destroy the networks providing advanced weaponry and training to our enemies in Iraq."

Bush's comments refer "strictly to operations inside the territory of Iraq, not crossing the border," Gates said, later adding that "any kind of military action inside Iran itself, that would be a very last resort."

Besides McCain, others on the Armed Services panel voicing support for Bush included Lindsey Graham , R-S.C., who last year challenged the White House on its detainee policy; Joseph Lieberman of Connecticut, elected as an independent last fall when Democrats backed an anti-war candidate, and Republicans John Cornyn of Texas, Jeff Sessions of Alabama and Saxby Chambliss of Georgia.

Graham urged "that we not have a political stampede to declare the war lost when it's not yet lost, or to embrace strategies that would lead to defeat."

The loudest objections to Bush's plan were expressed by Democratic Sens. Robert Byrd of West Virginia and Edward Kennedy of Massachusetts.

"In his speech to the nation, the president threatened that starting to bring our troops home would mean new terrorist threats to our homeland," said Byrd. "That's exactly the same sales job that was used to justify the start of this misguided war ¡ª that Saddam Hussein was planning for the day in which he would unleash weapons of mass destruction on our cities."

Republican Sens. John Warner of Virginia and Susan Collins of Maine said they were gravely concerned about the fate of Iraq. Collins asked Gates and Pace why the administration thinks the plan will work when past attempts have failed.

Warner said the goal must be to keep Iraq from being "scattered to the winds" in the region.