President Bush's nominee to lead the Iraq war for the United States told a Senate panel Tuesday that there will be "tough days" ahead in Iraq amid criticism of the White House plan for a change of course in the war-torn country.

"None of this will be rapid," Lt. Gen. David Petraeus told the Senate Armed Services Committee. "The way ahead will be neither quick nor easy."

Bush picked Petraeus to replace Army Gen. George Casey, who is retiring. Petraeus is likely to win Senate confirmation as commander of the multinational force in Iraq. Petraeus is a former division commander and was once the head of the Iraqi training mission.

Petraeus' appearance before the Senate panel comes hours before Bush prepares to deliver his sixth State of the Union address.

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Bush's Jan. 10 announcement ordered an additional 21,500 U.S. troops in Iraq to respond to sectarian violence and give the country's government another chance to take over its own security.

Senators grilled Petraeus on Bush's plan for a change of course in Iraq. Sen. Hillary Clinton, D-N.Y., said he is being sent to oversee a policy that "doesn't represent your experience or advice."

"You wrote the book, general, but the policy is not by the book. You are being asked to square the circle," Clinton said.

Clinton said that during her recent trip to Iraq, she found that Iraqis don't want Americans there.

"They cannot continue to rely on the blood and treasure of Americans any longer," Clinton said.

Sen. John McCain, R-Ariz., who supports Bush's plan, asked Petraeus how long the surge of troops could last.

"I am keenly aware of the strain" on the Army and Marine Corps, Petraeus said, adding that he welcomes Bush's proposal to increase the size of the land forces over the next five years.

Asked by McCain how soon he thought he would know whether the new strategy was working, Petraeus said, "We would have indicators at the least during the late summer."

In response to a question by Sen. Carl Levin, the chairman of the committee who opposes Bush's plan, Petraeus said the troop surge could be stopped if the Iraqi government is unable to provide thousands more Iraqi troops.

Earlier, the nominee said there were no "specific conditions" the Iraqis must meet in order to keep the flow of U.S. forces moving. The last of five additional U.S. brigades are scheduled to arrive in the Iraqi capital in May; the first got there just days ago.

Petraeus said that in the event Iraqis do not meet their commitments, he would consult with Defense Secretary Robert Gates on how to respond.

He said he would not have accepted the nomination to take command in Baghdad if he did not believe Bush's plan could work.

Petraeus has held a wide range of responsibilities during his 32 years in the Army, including in Bosnia, Germany, Italy and more recently in Iraq.

He told the Senate committee that he believes ethnic divisions in Iraq are not as severe as in Bosnia, where U.S. and NATO forces intervened in the 1990s to stop a civil war. He noted, however, that Iraq's divisions got worse in 2006.

On Monday, some Republican leaders on Capitol Hill offered a new resolution disapproving Bush's troop-surge plan that they say is less confrontational than a draft proposal circulated among Democrats.

The GOP measure is sponsored by, among others, Sen. John Warner, R-Va., the ranking Republican on the Senate Armed Services Committee who was once solidly in the president's corner.

Meanwhile, House Minority Leader John Boehner sent a letter to Speaker Nancy Pelosi asking for her support in building a new committee for Iraq oversight.

In his letter to Pelosi, he said the existing committee system responsible for Iraq policy in the House — primarily the House Armed Services, Foreign Relations and Appropriations committees — is too disparate to focus exclusively on the issue at hand.

FOX News' Trish Turner and The Associated Press contributed to this report.