Tense exchanges percolated throughout Capitol Hill Thursday as Defense Secretary Donald Rumsfeld (search) fended off calls for his resignation, rejected calls for a timetable for withdrawal from Iraq and urged patience in the face of continued terror attacks there.

Testifying before the Senate Armed Services Committee, Rumsfeld and the top military brass in Iraq and at the Pentagon outlined the challenges for that country and emphasized the positive steps being made there:

"Timing in war is never predictable. There are never guarantees," Rumsfeld said. "Those who say we are losing this war are wrong. We are not."

Some of the toughest criticism so far has come from Sen. Ted Kennedy (search), D-Mass., who laid out a stinging assessment of Rumsfeld's job performance, listing what have become the greatest hits for critics of the secretary.

Kennedy accused the secretary of wrongly sending soldiers to war without armor, said Rumsfeld exaggerated the successes in Iraq and charged he mismanaged the war and created an impossible situation.

"I'm talking about misjudgments, gross errors and mistakes. Those are on your watch. Isn't it time for you to resign?" Kennedy asked. "Our troops deserve better, the American people deserve better. They deserve competency and they deserve facts. In baseball, it's three strikes and you're out. What is it for the secretary of defense?"

Rumsfeld replied: "I've offered my resignation to the president twice and he's decided that he would prefer that he not accept it. That is his call."

Kennedy also got the attention of the panel when he called the situation in Iraq a "seemingly intractable quagmire ... with no end in sight."

"Well, that is quite a statement," Rumsfeld responded. "First, let me say there isn't a person at this table who agrees that we're in a quagmire and there isn't an end in sight."

"The presentations today have been very clear. They've been forthright. The suggestion by you that people like me or others are painting a rosy picture is false," Rumsfeld added, repeating an oft-made complaint that his words are often taken out of context and used against him.

Gen. George Casey, the top commander in Iraq, called Kennedy's description of the situation as a quagmire a "misrepresentation of the facts" and suggested it was an insult to the Iraqi people.

"You have an insurgency with no base of support, and Iraqi security forces are fighting and dying for their country every day. That is not a quagmire," Casey said.

Despite Casey's defense, Gen. John Abizaid (search) acknowledged that the Iraqi insurgency was undiminished over the past six months.

"I believe there are more foreign fighters coming into Iraq than there were six months ago," he said, adding that the overall strength of the insurgency is "about the same" as it was 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," Abizaid said.

Several Democratic senators suggested that Iraq's security forces are also not up to the task of defending themselves, another contention disputed by Rumsfeld.

"To be sure they're not like U.S. forces, (are) never going to be ... there isn't an army, navy in the world that is comparable to the U.S. military but that does not mean they're not capable," he said.

"Success will not be easy and it will require patience. ... But consider what has been accomplished in 12 months," Rumsfeld added, citing elections in January, economic improvements and an increasingly improving security force.

Some panel Democrats suggested that if the Iraqis do not design a constitution on time, the United States will have to review its options, including whether it should set up a timetable for withdrawal.

"Failure to adopt a constitution ... shows a lack of will," said Sen. Carl Levin of Michigan, the committee's senior Democrat.

"We must demonstrate to the Iraqis that our willingness to bear the burden...has limits," Levin said. However, like President Bush and others in his administration, Levin said he did not support a U.S.-set timetable for a U.S. exit strategy at this time.

"That policy would be counterproductive," he said.

"Leaving before the task is complete would be catastrophic," Gen. Richard Myers (search), chairman of the Joint Chiefs of Staff, told the panel.

Committee Chairman John Warner, R-Va., praised President Bush for "steady and unflinching resolve."

"Our great nation has an enormous capacity for sacrifice and hardship when we understand the cause is just," he said.

But with more than 1,700 dead U.S. soldiers and sailors in Iraq since the start of war in March 2003, even some Republicans expressed impatience with the challenge.

"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."

With public opinion polls showing a drop in support for the war effort, patience appears to be one of the chief challenges for the administration, said Rick Davis, a former campaign manager for Sen. John McCain, R-Ariz.

"Left to their own devices, the public will" be impatient, he told FOX News. "I think [terrorists] are encouraged by public opinion that says [Americans] don't see the war as popular, but I also think they see what's going on on the ground, which is no let up in this protracted war. We're in to win and we're going to do what we have to" to make it so.

FOX News' Greg Kelly and The Associated Press contributed to this report.