The Bush administration’s top military and diplomatic officials in Iraq faced the next president of the United States Tuesday in a highly anticipated appearance expected to shape the next phase of the Iraq debate.

In testimony before two Senate panels, Gen. David Petraeus and Ambassador Ryan Crocker found themselves in a rare public forum answering questions from all three presidential candidates — and listening to speeches.

As expected, Sen. John McCain (Ariz.), the presumptive GOP nominee, stood solidly behind Petraeus and Crocker’s call that premature withdrawal of troops could reverse incremental security gains on the ground.

Meanwhile, Sens. Barack Obama (Ill.) and Hillary Rodham Clinton (N.Y.), who are battling each other for the Democratic nomination, used the day’s events to drive home the case that the war is not going well. At one point, Clinton rebuked Crocker for suggesting that the Iraqi parliament would have an opportunity to approve an agreement regarding the U.S. commitment there, and the U.S. Congress would not.

For the most part, the administration officials stood by the position that troops should return home based only on the conditions on the ground. They asserted that an agreement under development on the United States’ long-term commitment in Iraq would not need to be approved by the Senate to take force, frustrating Democrats.

“We have our teeth into [al Qaeda’s] jugular and we need to keep it there,” Petraeus said at an afternoon hearing before the Senate Foreign Relations Committee, prompting groans from anti-war protesters in a packed audience.

The testimony comes at a sensitive time in the debate. Republicans say that the recent decrease in violence buoys their arguments that troops need to remain in Iraq to finish off al Qaeda. Democrats contend that the security improvements have been marginalized by Iraq’s much-criticized operation in Basra taking on Muqtada al-Sadr, who threatened Tuesday to call off a seven-month cease-fire.

“What has been achieved is substantial, but it is also reversible,” Crocker told the Armed Services Committee.

As the most senior Republican on the Armed Services Committee, McCain had the opportunity to tout his position far ahead of his more junior presidential rivals.

“Should the United States instead choose to withdraw from Iraq before adequate security is established, we will exchange for this victory a defeat that is terrible and long-lasting,” said McCain, who advocates keeping troops in Iraq until the country stabilizes. “Al Qaeda in Iraq would proclaim victory and increase its efforts to provoke sectarian tensions, pushing for a full-scale civil war that could descend into genocide and destabilize the Middle East.”

McCain’s opening statement was briefly interrupted when a protester shouted: “There is no military solution!”

“I’ve had this experience previously, Mr. Chairman,” McCain quipped.