"(We) have every confidence that in the event force is to be used in Iraq, that we can do what needs to be done using conventional capabilities," he told the committee.
Rumsfeld said the United States has never promised not to use nuclear weapons, as a matter of policy.
"Our policy historically has been generally that we will not foreclose the possible use of nuclear weapons if attacked," he said. "Does the department have an obligation and have they in successive administrations of both political parties had procedures whereby we would conceivably use nuclear weapons? Yes."
Rumsfeld tacitly acknowledged the Pentagon has contingency plans that involve nuclear weapons, both strategic and theater, or low-yield weapons -- but the threshold for using them is very high.
"As a part of contingency planning, the United States has, in my adult lifetime, always had contingency plans to do a variety of things," he said. "And it seems to me that if one looks at our record, we went through the Korean War, we went through the Vietnam War, we've gone through the war on terror, and we've not used nuclear weapons. That ought to say something about the threshold with respect to nuclear weapons."
The threshold for using low-yield theater weapons might be getting lower, however. The classified Nuclear Posture Review written last year suggests they might be useful against hardened, underground facilities that conventional weapons can't penetrate.
"Nuclear weapons could be employed against targets able to withstand non-nuclear attack (for example, deep underground bunkers or bio-weapon facilities)," the document stated.
In January, White House Chief of Staff Andrew Card told NBC's "Meet the Press" that President George W. Bush has not ruled out the use of nuclear weapons against Iraq.
"Should Saddam Hussein have any thought that he would use a weapon of mass destruction, he should anticipate that the United States will use whatever means necessary to protect us and the world from a holocaust," Card said.
Asked if he specifically meant nuclear weapons, he responded: "I'm not going to put anything on the table or off the table, but we have a responsibility to make sure Saddam Hussein and his generals do not use weapons of mass destruction."
Rumsfeld told reporters after testimony to the same committee in September 2002 that the use of nuclear weapons -- even in retaliation for chemical or biological weapon attacks on the battlefield -- is not being considered.
"That is not even a thought," he said.
Rumsfeld and Chairman of the Joint Chiefs of Staff Gen. Richard Myers said in September the deterrent to the use of weapons of mass destruction is the threat of facing war crimes charges.
"Clearly (Iraqi) people who defect, people who assist (the United States) are particularly favored. People who do not assist are not. People who do assist in weapons of mass destruction are particularly not favored," Rumsfeld said at the time.
The United States is the only nation that has ever used nuclear weapons in battle, during World War II against two cities in Japan.