"We have now set the bar so low, that modest improvement ... is considered success. And it's not," he said.
Joseph Biden, another Democrat senator and presidential hopeful for 2008, opened the hearing by saying the so-called "surge" of US troops, begun in February, had not provided the political breakthrough it was supposed to.
"The American people will not support an infinite war whose sole remaining purpose is to prevent the situation in Iraq from becoming worse than it is today," Biden said.
Petraeus repeated his belief that the "surge" was going well.
Rob Reynolds, Al Jazeera's correspondent in Washington, said although the testimony was the same, the exchanges with senators was "far sharper".
As well as the expected criticism from the Democrats, who control both houses of congress, scepticism over the policy pursued by George Bush was expressed within his own party.
Senator Richard Lugar, the most senior Republican on the senate foreign relations committee, told the hearing: "Some type of success in Iraq is possible, but as policy makers, we should acknowledge that we are facing extraordinarily narrow margins for achieving our goals."
"Iraq is still under foreign occupation and Iraqis continue to die in great numbers"
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Lugar has called for a sizable withdrawal of US troops in the coming months, a plan that runs counter to the recommendation by Petraeus that the US withdraw the 30,000 extra troops deployed this year, but maintain at least 130,000 troops.
Petraeus has said the "surge" of troops was aimed at bringing security that would provide a platform of political progress and reconciliation in Iraq.
Chuck Hagel, a senior Republican critic of the war, said he saw no reason for the positive assessments of Petraeus and Crocker.
He said: "Are we going to continue to invest American blood and treasure at the same rate we are doing now ... for what?
"The president said let's buy time. Buy time? For what?"
Petraeus has previously stressed that his testimony was his own and had not been shown to anyone in the White House or Pentagon amid suggestions from Democrats prior to the hearing that it would be politicised.
But in an editorial on Tuesday, The New York Times described the general's testimony as "another of the broken promises and false claims we have heard from Mr Bush".
Several protesters who heckled the hearing were ejected from the chamber in a repeat of similar scenes the day before.
The Iraqi government, which has come under fierce criticism for not making progress on political benchmarks, welcomed Petraeus' report on Monday.
Mowaffaq al-Rubaie, Iraq's national security adviser, said that in the near future he expected "our need will be diminished for the multinational forces to conduct direct combat operations".