The top U.S. military commander in Iraq told a Senate panel Tuesday he has recommended a 45-day moratorium on troop drawdowns following the removal in July of the last unit sent as part of President Bush's troop surge, which began last year.

Gen. David Petraeus, speaking before the Senate Armed Services Committee, said "substantial" progress has been made in securing Iraq since the beginning of the troop surge, and despite recent fighting, said the country has improved since his last appearance before Congress last September.

Petraeus' testimony was met with pushback from Democrats who said the plan was indefinite, and does not move fast enough to begin bringing troops home and shift responsibilities over to Iraqis.

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

Senate Foreign Relations Committee Chairman Joe Biden likened the ongoing fighting in Iraq to treading water during the afternooon session before his committee.

"Fifteen months into the surge, we've gone from drowning to treading water. We're still spending $3 billion every week and we're still losing -- thank God it's less -- but 30 to 40 American lives every month," Biden said.

"We can't keep treading water without exhausting ourselves. But that's what the president seem to be asking us to do. He can't tell us when or even if Iraqis will come together politically. He can't tell us when or even if we will draw down below the pre-surge level. He can't tell us when or even if Iraq will be able to stand on its own two feet," Biden said.

Petraeus, however, warned that progress is still "fragile" and "reversible," and therefore said he would not set a firm period on how the military will decide further troop movements.

He said that at the end of the 45-day period, "We will commence a process of assessment to examine the conditions on the ground, and over time determine when we can make recommendations for further reductions.

"This process will be continuous, with recommendations for further reductions made as condition permit," Petraeus continued. "This approach does not allow establishment of a set withdrawal timetable, however, it does provide the flexibility those of us on the ground need to preserve the still fragile security gains our troopers have fought so hard and sacrificed so much to achieve."

Petraeus and U.S. Ambassador to Iraq Ryan Crocker were delivering their report on military and political progress in Iraq. Following the morning hearing, they were scheduled to appear in the afternoon before the Senate Foreign Relations Committee, and on Wednesday they were to return, testifying to two House committees.

Petraeus said it remains in the United States' interest to stay in Iraq.

"It clearly is in our national interests to help Iraq prevent the resurgence of Al Qaeda in the heart of the Arab world, to help Iraq resist Iranian encroachment on its sovereignty, to avoid renewed ethno-sectarian violence that could spill over Iraq's borders and make the existing refugee crisis even worse, and to enable Iraq to expand its role in the regional and global economies," he said.

Petraeus' plan would allow the five extra brigades ordered to Iraq last year to withdraw by July without ordering their replacement. Also this week, possibly on Thursday when Bush addresses the nation on the war, the administration plans to announce that soldiers will spend no longer than 12 months at a time in combat, a decrease of three months in current combat tours.

• 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)

Petraeus and Crocker noted the continuation of a number of developments that shed a positive light on the U.S troop presence in Iraq. Violence is down to 2005 levels. Civilian deaths are back to a level before the February 2007 bombing of the Shiite shrine in Samarra. Iraqis have made a number of political steps including passing a budget, a de-Baathification law, they are on the way to passing an oil-sharing law, and its GDP is growing.

"As Iraq is now earning the financial resources it needs for bricks-and-mortar construction through oil production and export, our assistance has shifted ... ," Crocker said, proclaiming, "The era of U.S.-funded major infrastructure projects is over."

That declaration might have pleased some Democrats, who are believe the United States has not transferred responsibility to Iraqis fast enough.

To open the hearing, Armed Services Committee Chairman Carl Levin, D-Mich., said recent fighting in Iraq has cast new questions on the effectiveness of the U.S. troop surge, and the United States must move toward shifting the burden of fixing Iraq over to the Iraqis.

"The purpose of the surge as announced by President Bush last year ... has not been achieved," said Levin, chairman of the Senate Armed Services Committee.

Levin said the recent outbreak of fighting in Basra "leads many of us to once again challenge Bush's policies." He called Iraqi Prime Minister Nouri al-Maliki's plans "haphazardly" developed, and said the United States must "force that government to take responsibility for their own future."

The top Republican on the Senate Foreign Relations Committee, Sen. Dick Lugar, R-Ind., was expected to issue some criticism of the current military plan, saying that simply appealing for more time is insufficient.

"Simply appealing for more time to make progress is insufficient. The debate over how much progress we have made and whether we can make more is less illuminating than determining whether the administration has a definable political strategy that recognizes the time limitations we face and seeks a realistic outcome designed to protect American vital interests," according to Lugar's prepared remarks.

"Unless the United States is able to convert progress made thus far into a sustainable political accommodation that supports our long-term national security objectives in Iraq, this progress will have limited meaning," he said.

Under questioning by Levin, Petraeus said he could not predict when troop reductions would be resumed or how many U.S. troops were likely to remain in Iraq by the end of this year. There currently are 160,000 U.S. troops in Iraq, and the Pentagon has projected that when the scheduled troop withdrawals are completed in July there will be about 140,000 troops there.

Levin reminded Petraeus that Defense Secretary Robert Gates has said a "pause" in troop reductions should be "brief." Noting that Petraeus used neither word, Levin said the Petraeus plan amounted to an open-ended suspension.

"What you've given to your chain of command is a plan which has no end to it," Levin said. He asked Petraeus when he would be in position to recommend further troop cuts, once the 45-day evaluation period ends in September.

"It could be right then, or it could be longer," the general said. He declined to be pinned down, saying he would recommend further cuts when conditions were right.

During the exchange with Levin, the hearing was briefly interrupted by one protester repeatedly shouting, "Bring them home!" The protester was removed from the hearing room by two members of the Capitol Police force.

Sen. John McCain, R-Ariz., however, said he believes Iraq is showing signs of success, and warned against pulling out too many troops too early.

"Since the middle of last year, sectarian and ethnic violence, civilian deaths, and deaths of coalition forces have all fallen dramatically. This improved security environment has led to a new opportunity, one in which average Iraqis can, in the future, approach a more normal political and economic life," McCain said.

"Today, it is possible to talk with real hope and optimism about the future of Iraq and the outcome of our efforts there," he said, warning against a "reckless and irresponsible withdrawal of our forces at the moment when they are succeeding."

He said pulling troops out too soon would mean exchanging for victory "a defeat that is terrible and long lasting."

The division between Democrats and Republicans was clear among the first group of senators Monday, with Democrats lining up skeptical of the administration's claims in Iraq, and Republicans sympathetic to their report.

Sen. Edward Kennedy, D-Mass., used a line of questioning to show he believed U.S. forces were wrongly being drawn into sectarian fighting, rather than maintaining the fight only against Al Qaeda.

"Now, you mentioned that the battle in Basra was to take on the criminals and extremists. Aren't we in there to battle Al Qaeda?" Kennedy asked Petraeus.

Petraeus answered that Basra is predominantly Shiite -- not Sunni, the religious sect of Islam to which Al Qaeda members belong.

"But we're over in Iraq to take on Al Qaeda, and here we've got the Maliki government moving in to battle intersectarian violence that's taking place, which many believe can enhance the possibilities of civil war," Kennedy said.

Later on, Sen. Lindsay Graham, R-S.C., told Petraeus that by the way some people were putting it, "You should be fired."

Graham quickly pointed out that wasn't the case, and with the military, political and economic gains being made in Iraq, "If I could promote you to five-star general, I would."

Statistics show that while violence in Iraq is still down substantially, the slow withdrawal of U.S. troops that began in December has been accompanied by spikes in both deaths and attacks.

The internal strife was underscored by a rise in ethno-sectarian violence between Iraqis in March, the first such monthly increase since last July.

Defense officials also warned Monday of another likely spike in attacks this week, as U.S. forces strike back at militia fighters in Sadr City. And officials also said there are indications that Al Qaeda is looking for an opportunity to reassert its influence in the Baghdad region.

Petraeus said a recent cease-fire negotiated by Iraqi leaders and radical Shiite cleric Moqtada al-Sadr as well as a cease-fire in Karbala last August show that Iraqi security forces are doing better than would be suggested by the planning and losses that occurred.

He added that the cease-fires aimed "to avoid further damage to the image of the Sadr movement, which of course is supposed to care for the down-trodden and is a religiously inspired movement but which has been hijacked in some cases by militias and in fact other elements have used it to cloak their activities as well," Petraeus said.

If Petraeus' proposal comes to pass, as many as 140,000 troops could be in Iraq when voters head to the polls in the U.S. this fall.

Democrats contend that the existing approach guarantees an open-ended commitment to a $10-billion-a-month war as the economy at home is faltering. They say the lack of political progress made in Iraq, as well as the recent spike in violence in Basra, indicates the U.S. troop buildup has failed.

"We need a strategy that will clearly shift the burden to the Iraqis, that'll begin to take the pressure off our forces, begin to allow us to respond to other challenges in the region and worldwide," said Sen. Jack Reed, D-R.I., a member of the Armed Services Committee.

Democrats also acknowledge that they are more or less helpless in trying to force Bush's hand on the war. While anti-war legislation has been able to pass the House, it repeatedly sinks in the Senate, where Democrats lack the 60 votes needed to overcome procedural hurdles.

They contend, however, that come fall dissatisfied voters will head to the polls and put more Democrats in power, possibly including an anti-war president. In a recent FOX News poll, only 30 percent said they approve of the job Bush is doing.

Indeed, Tuesday's hearings are expected to be about as much as the presidential elections as they are about the state of Iraq. The three major candidates for president are on the committees for which Petraeus is providing testimony.

McCain, the ranking Republican on the Armed Services Committee, and Sen. Hillary Rodham Clinton, D-N.Y., a member of the panel, are expected to use the morning committee hearing to showcase their opposing views on the war. Sen. Barack Obama, D-Ill., will get his chance later that afternoon as member of the Foreign Relations Committee.

For now, Petraeus faces a dramatically different political landscape than last fall when support for the war had been eroding steadily among Republicans. Petraeus' testimony helped shore up GOP defections at the time. And since then, a significant drop in violence has helped stave off legislation ordering troops home.

In addition to insisting that troops must stay in Iraq to fight the terrorists, which has been the party line for some time, Republicans are expected to talk more about the need for a comprehensive political settlement among Baghdad politicians. They believe that this tracks more closely with the voters' views that the U.S. commitment cannot be indefinite.

The Associated Press contributed to this report.