WASHINGTON – The top American commander in the Persian Gulf told Congress yesterday that the Iraqi insurgency has not grown weaker over the past six months, despite a claim by Vice President Cheney that it was in its “last throes.”

General John Abizaid’s testimony came at a contentious Senate Armed Services Committee hearing at which Defense Secretary Rumsfeld clashed with members of both parties, including a renewed call by Senator Kennedy of Massachusetts for him to step down.

Citing what he called repeated “gross errors and mistakes” in the American military campaign in Iraq, Mr. Kennedy told Mr. Rumsfeld: “In baseball, it’s three strikes, you’re out. What is it for the secretary of defense?”

“Isn’t it time for you to resign?” Mr. Kennedy asked.

“I’ve offered my resignation to the president twice,” Mr. Rumsfeld shot back, saying that President Bush had decided not to accept it. “That’s his call,” he said.

Mr. Kennedy has called for Mr.Rumsfeld’s resignation before. Mr. Rumsfeld, General Abizaid, and other top defense officials were grilled on the future presence of American troops in Iraq. As to proposals from some lawmakers to set a timetable for withdrawal, Mr. Rumsfeld said: “That would be a mistake.”

But even some Republicans expressed open skepticism with how America is conducting the war in Iraq, with American deaths now surpassing 1,700 since the war began in March 2003.

“Public support in my state is turning,” Senator Graham, a Republican of South Carolina, said. “People are beginning to question. And I don’t think it’s a blip on the radar screen. We have a chronic problem on our hands.”

Committee Democrats and some Republicans on the panel accused the administration of being overly optimistic, including Mr. Cheney’s “last throes” observation about the insurgency.

General Abizaid told the panel: “I believe there are more foreign fighters coming into Iraq than there were six months ago.”As to the overall strength of the insurgency, General Abizaid said it was “about the same” as six months ago.

“We see good progress in both Iraq and Afghanistan… But we are realistic. And we know that great change is often accompanied with violence. We are not trying to paint a rosy picture,” General Abizaid said. Told by Senator Levin of Michigan, the committee’s senior Democrat, that his assessment directly contradicted Mr. Cheney’s, General Abizaid said: “I don’t know that I would make any comment about that other than to say there’s a lot of work to be done… I gave you my opinion.”

In a CNN interview last month, Mr. Cheney said: “I think they’re in the last throes, if you will, of the insurgency.”

“The fact is that the insurgency has not weakened,” Mr. Levin said. “Our men and women in uniform are serving with great honor. They deserve an objective assessment of the situation in Iraq. They deserve a clear layout of the next steps there. They’re not getting either from the administration.”

Mr. Rumsfeld voiced strong opposition to congressional calls for an exit strategy with a timetable for withdrawing American troops. “Timing in war is never predictable. There are never guarantees,” Mr. Rumsfeld said. “Those who say we are losing this war are wrong. We are not.” Testifying on the progress in training Iraq’s own security forces, Mr. Rumsfeld said these forces have “a way to go,” but progress was being made.

The administration contends that Iraqis must be able to defend their own country against a lethal insurgency before a timeline for bringing home troops can be considered.

But progress has been slower than expected. In recent weeks, insurgents have increasingly targeted Iraqi security forces. And American casualties, war spending, and public skepticism continue to climb. “Leaving before the task is complete would be catastrophic,” the chairman of the Joint Chiefs of Staff, General Richard Myers, told the panel.

Mr. Levin said the Bush administration should tell the Iraqis that if they do not meet their deadline for drafting a constitution – August 15, with a possible six-month extension – America will consider setting a timetable for troop withdrawals.