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

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

Citing what he called repeated “gross errors and mistakes” in the U.S. military campaign in Iraq, Kennedy told 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?” Kennedy asked.

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

Kennedy has called for Rumsfeld’s resignation before.

Rumsfeld, Abizaid and other top defense officials were grilled on the future presence of U.S. troops in Iraq. As to proposals from some lawmakers to set a timetable for U.S. withdrawal, Rumsfeld said: “That would be a mistake.”

Defense Secretary Donald Rumsfeld, right, shakes hands with Sen. Edward Kennedy, D-Mass., on Capitol Hill Thursday, during Rumsfeld's appearance before the Senate Armed Services Committee.

But even some Republicans expressed open skepticism with U.S. policy in Iraq, with U.S. deaths now surpassing 1,700 since the war began in March 2003.

“Public support in my state is turning,” said Sen. Lindsey Graham, R-S.C. “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 Cheney’s Memorial Day “last throes” observation about the insurgency.

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, 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,” Abizaid said.

Carl Levin, the top-ranking Democrat on the committee, 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 – the United States will consider setting a timetable for troop withdrawals.

“We must demonstrate to the Iraqis that our willingness to bear the burden : has limits,” Levin said.