The top U.S. commander in Iraq told Congress Tuesday that hard-won gains in the war zone are too fragile to promise any troop pullouts beyond this summer, holding his ground against impatient Democrats and refusing to commit to more withdrawals before President Bush leaves office in January.

Army Gen. David Petraeus painted a picture of a nation struggling to suppress violence among its own people and to move toward the political reconciliation that Bush said a year ago was the ultimate aim of his new Iraq strategy, which included sending more than 20,000 extra combat troops.

Security is getting better, and Iraq's own forces are becoming more able, Petraeus said. But he also ticked off a list of reasons for worry, including the threat of a resurgence of Sunni or Shiite extremist violence. He highlighted Iran as a special concern, for its training and equipping of extremists.

In back-to-back appearances before two Senate committees, Petraeus was told by a parade of Democrats that, after five years of war, it was past time to turn over much more of the war burden to the Iraqis. Those senators said Iraq will not attain stability until the United States makes the decision to begin withdrawing in large numbers and forces the Iraqis to settle their differences.

Photo Essay: Gen. Petraeus, Ambassador Crocker deliver new Iraq report

Republican Sen. George Voinovich of Ohio, a longtime critic of the administration's war strategy, told Petraeus: "The American people have had it up to here."

Petraeus responded, "I certainly share the frustration."

But when it came to promising or predicting a timetable for further withdrawals, Petraeus didn't budge. He said he had recommended to Bush that he complete, by the end of July, the withdrawal of the 20,000 extra troops. Beyond that, the general proposed a 45-day period of "consolidation and evaluation," to be followed by an indefinite period of assessment before he would recommend any further pullouts.

The Petraeus plan, which Bush is expected to embrace, reflects a conservative approach that leaves open the possibility that roughly 140,000 U.S. troops could remain in Iraq when the president leaves office next year.

On Thursday Bush will make a speech about the war, now in its sixth year, and his decision about troop levels.

• Gen. Petraeus' Testimony (.pdf)

• Ambassador Crocker's Senate Armed Services Committee Testimony (.pdf)

• Ambassador Crocker's Senate Foreign Relations Committee Testimony (.pdf)

• Testimony Handout Packet (.pdf)

In exchanges with several senators, Petraeus refused to say when he thought it would be safe to resume troop reductions beyond July without risking "fragile and reversible" security gains.

Asked Sen. Carl Levin, chairman of the Armed Services Committee: "Could that be a month, could that be two months?"

Petraeus began to respond: "Sir, it could be less than that. It could be. ..."

Levin: "Could it be more than that?"

Petraeus: "It could be more than that. Again, it's when the conditions are met that we can make a recommendation for further reductions."

Levin: "Could it be three months?"

Petraeus: "Sir, again, at the end of the period of consolidation and evaluation. ..."

On they went in the same vein, even after a demonstrator — "Bring them home! Bring them home!" — interrupted the hearing and was escorted out.

When Sen. Evan Bayh, D-Ind., started in again later, Petraeus said it would defy logic to establish a timetable before knowing what conditions will be like this summer.

"If you believe as I do — and the commanders on the ground believe — that the way forward on reductions should be conditions-based then it is just flat not responsible to try to put down a stake in the ground and say this is when it would be or that is when it would be," Petraeus said.

One of three senators who could be the new president by January, Hillary Rodham Clinton, said much earlier, not in a response to Petraeus, that she disagreed with those who criticized lawmakers who are calling for an orderly withdrawal.

"Rather, I think it could be fair to say that it might well be irresponsible to continue the policy that has not produced the results that have been promised time and time again at such tremendous cost to our national security and to the men and women who wear the uniform of the United States military," she said.

Sen. Barack Obama, her rival for the Democratic presidential nomination, told Petraeus that while he wants U.S. troops out of Iraq he would not initiate a precipitous withdrawal. And he said talking regularly to the Iranians is critical to getting to the point where it would be safe to end American involvement.

"I do not believe we are going to be able to stabilize the situation without them," Obama said.

War supporter John McCain, who will be the GOP nominee, said: "Our goal — my goal — is an Iraq that no longer needs American troops. And I believe we can achieve that goal, perhaps sooner than many imagine. But I also believe that to promise a withdrawal of our forces, regardless of the consequences, would constitute a failure of political and moral leadership."

Petraeus said his plan is supported by Adm. Michael Mullen, chairman of the Joint Chiefs of Staff, who has been notably public in his expressions of concern that the heavy commitment of troops in Iraq has limited U.S. military options elsewhere and has put enormous strain on troops and their families.

Petraeus made no mention of reducing soldiers' tours of duty in Iraq from the current 15 months to 12 months, but the administration is expected to announce a decision to do that this week. It would take effect this summer, coinciding with the completion of the drawdown to 15 combat brigades in Iraq.

Petraeus said the recent flare-up of violence in Basra, in Baghdad and elsewhere points up the importance of the cease-fire declared last year by anti-American cleric Muqtada al-Sadr and highlighted the role Iran allegedly plays in funding and training Shiite militias through cells the U.S. military calls "special groups."

"Unchecked, the special groups pose the greatest long-term threat to the viability of a democratic Iraq," Petraeus said.

Testifying beside Petraeus was Ryan Crocker, the U.S. ambassador to Baghdad, who also focused on the violence in Basra, where Prime Minister Nouri al-Maliki dispatched Iraqi security forces to combat Shiite militias.

"Taken as a snapshot, with scenes of increasing violence, and masked gunmen in the streets, it is hard to see how this situation supports a narrative of progress in Iraq," Crocker said. "There is still very much to be done to bring full government control to the streets of Basra and eliminate entrenched extremist, criminal, and militia groups. When viewed with a broader lens, the Iraqi decision to combat these groups in Basra has major significance."