With his often-combative defense of the war in Iraq, Rumsfeld had been the administration's face of the conflict. He became more of a target - and more politically vulnerable - as the war grew increasingly unpopular at home amid rising violence and with no end in sight.

Sen. John Warner, R-Va., chairman of the Armed Services Committee, said he hopes to hold Gates' confirmation hearings in time for the Senate to approve his nomination this year. But Senate Democratic leader Harry Reid of Nevada, whose party would control the Senate next year should it win the remaining undecided race in Virginia, said he had questions about Gates' ties to the Iran-Contra scandal of the Reagan administration.

Gates ran the CIA under the first President Bush during the first Gulf war. He retired from government in 1993.

He joined the CIA in 1966 and is the only agency employee to rise from an entry level job to become director. A native of Kansas, he made a name for himself as an analyst specializing in the former Soviet Union and he served in the intelligence community for more than a quarter century, under six presidents.

Numerous Democrats in Congress had been calling for Rumsfeld's resignation for many months, asserting that his management of the war and of the military had been a resounding failure. Critics also accused Rumsfeld of not fully considering the advice of his generals and of refusing to consider alternative courses of action.

Sen. Carl Levin of Michigan and Rep. Ike Skelton of Missouri - the top Democrats on the Armed Services committees - said the resignation would be a positive step only if accompanied by a change in policy.

"I think it is critical that this change be more than just a different face on the old policy," Skelton said.

Rumsfeld, 74, has served in the job longer than anyone except Robert McNamara, who became secretary of defense during the Kennedy administration and remained until 1968. Rumsfeld is the only person to have served in the job twice; his previous tour was during the Ford administration.

Rumsfeld had twice previously offered his resignation to Bush - once during the Abu Ghraib prisoner abuse scandal in spring 2004 and again shortly after that. Both times the president refused to let him leave.

Gates took over the CIA as acting director in 1987, when William Casey was terminally ill with cancer. Questions were raised about Gates' knowledge of the Iran-Contra affair, and he withdrew from consideration to take over the CIA permanently. Yet he stayed on as deputy director.

Then-National Security Adviser Brent Scowcroft, who has been a critic of the younger Bush's policies, asked Gates to be his deputy in 1989 during the administration of Bush's father. The elder President Bush, a former CIA director himself, asked Gates to run the CIA two years later.

Gates won confirmation, but only after hearings in which he was accused by CIA officials of manipulating intelligence as a senior analyst in the 1980s.

Melvin Goodman, a former CIA division chief for Soviet affairs, testified that Gates politicized the intelligence on Iran, Nicaragua, Afghanistan and the Soviet Union. "Gates' role in this activity was to corrupt the process and the ethics of intelligence on all of these issues," Goodman testified.

The Bush administration's use of intelligence on Iraq has been a central theme of criticism from Democrats who say the White House stretched faulty intelligence from US spy agencies to justify invading Iraq in 2003.

Gates has taken a much lower profile since leaving government. He joined corporate boards and wrote a memoir, "From The Shadows: The Ultimate Insider's Story of Five Presidents and How They Won the Cold War." It was published in 1996.

Gates is a close friend of the Bush family, and particularly the first President Bush. He became the president of Texas A&M University in August 2002. The university is home to the presidential library of the elder Bush.

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