Abizaid and Pace said they did not foresee a year ago that sectarian violence would be as high as it is now.

Abizaid said he believed Iraq would "move toward equilibrium in the next five years" with the right mix of political and military pressure. Bush has said he does not expect the last U.S. troops to leave during his presidency, which ends in January 2009.

"Shiite and Sunni are going to have to love their children more than they hate each other," Pace said. "The weight of that must be on the Iraqi people and the Iraqi government."

The Bush administration's handling of the war drew sharp rebukes from Democrats and some Republicans Thursday. Sen. John McCain likened the positioning of forces in Iraq to a game of "whack-a-mole," where generals try to curb violence in one area only to see it pop up somewhere else.

"It's very disturbing," said McCain, R-Ariz. "And if it's all up to the Iraqi military, General Abizaid, and if it's all up to them, then I wonder why we have to move troops into Baghdad to intervene in what is clearly sectarian violence."

Rumsfeld also sparred with Democratic senators over his handling of the war.

Sen. Hillary Rodham Clinton of New York told Rumsfeld he was "presiding over a failed policy" in Iraq, and asked him why lawmakers should believe his assurances that conditions in Iraq would improve.

"My goodness," Rumsfeld responded to her list of complaints; then he restated administration positions.

Clinton later told the Associated Press the president should accept Rumsfeld's resignation.

The generals' comments posed anew the question of what would happen if the Iraqi government crumbled and U.S. troops were left between competing armed militias. Sen. John W. Warner , who chairs the Armed Services Committee, said a civil war in Iraq would raise questions about the U.S. commitment there.

"I think we have to examine very carefully what Congress authorized the president to do in the context of a situation if we're faced with an all-out civil war and whether we have to come back to the Congress to get further indication of support," said Warner, R-Va.

In yet another sign of lawmakers' uncertainty of the situation in Iraq, Sen. Pat Roberts, R-Kan., on Thursday called for a revised intelligence estimate of Iraq, a document prepared by the intelligence community to give officials an unvarnished snapshot of the security situation.

Pace told McCain that U.S. troops are trained and equipped to respond to violence caused by ethnic strife, but their role would be limited.

"There's a difference between the kind of violence they have to handle and what will prevent that violence," Pace said. "And preventing that violence is very much the role of the political leaders in Iraq to solve, sir."

Later in the hearing, the generals expressed confidence that the Iraqi government was moving in the right direction.

"Am I optimistic whether or not Iraqi forces, with our support, with the backing of the Iraqi government, can prevent the slide to civil war? My answer is yes," Abizaid said.