McCain said that in contrast to Iraq, South Vietnam "never had a legitimate government in Saigon that the people believed in and trusted."

The Vietnam war also featured "superpower engagement in a huge way," McCain added. "We have a problem [in Iraq] with the Syrians, but nothing like what the Chinese and Russians were doing with the North Vietnamese," he said

Another difference, said McCain: "If we fail in Iraq, the results will be cataclysmic. You'll see factionalization and eventual Muslim extremism and terrorist breeding grounds that I believe will pose a direct threat to the security of the United States."

"The whole situation is just very, very different," the Arizona Republican argued.

Last week, Sen. Hagel encouraged opponents of the Iraq war by claiming, "We are locked into a bogged down problem not unsimilar, dissimilar to where we were in Vietnam."

McCain also declined an invitation from "Face the Nation" host Bob Schieffer to repeat his earlier criticism of Secretary of Defense Donald Rumsfeld.

"It does not help in any way for me to get into some kind of a fight with the secretary of defense, since he is there," McCain said, after Schieffer asked about his statement last year that he had "lost confidence" in Rumsfeld.

"We're in a crucial time in our history," McCain said. "I think that I will do everything in my power to work with the secretary of defense. His continuance - or non-continuance in office - is up to the president. And I'll support the president."

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