The Iraq war was the central issue of Rumsfeld’s nearly six-year tenure, and unhappiness with the war was a major element of voter dissatisfaction Tuesday and the main impetus for his departure. Even some Republican lawmakers became critical of war management, and growing numbers of politicians were urging Bush to replace Rumsfeld.

Bush said Robert Gates, 63, who has served in a variety of national security jobs under six previous presidents, would be nominated to replace Rumsfeld. Gates, currently the president of Texas A&M University, is a Bush family friend and a member of an independent group studying the way ahead in Iraq.

The White House hopes that replacing Rumsfeld with Gates can help refresh U.S. policy on the deeply unpopular war and perhaps establish a stronger rapport with the new Congress. Rumsfeld had a rocky relationship with many lawmakers.

“Secretary Rumsfeld and I agreed that sometimes it’s necessary to have a fresh perspective,’’ Bush said in the abrupt announcement during a news conference Wednesday.

In a later appearance at the White House with Rumsfeld and Gates at his side, Bush praised both men, thanked Rumsfeld for his service and predicted that Gates would bring fresh ideas.

Rumsfeld described the Iraq conflict as a “little understood, unfamiliar war that is complex for people to comprehend.’’ Upon returning to the Pentagon after appearing with Bush and Gates, Rumsfeld said it was a good time for him to leave.

“It will be a different Congress, a different environment, moving toward a presidential election and a lot of partisanship, and it struck me that this would be a good thing for everybody,’’ Rumsfeld told reporters.

There was little outward reaction among officials at the Pentagon, beyond surprise at the abrupt announcement.

Just last week Bush told reporters that he expected Rumsfeld, 74, to remain until the end of the administration’s term. Although Bush said his decision to replace Rumsfeld was not based on politics, the announcement of a Pentagon shake-up came after Tuesday’s voting.

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 amid rising violence and with no end in sight.

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.

Rumsfeld 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.