IMPEACH BUSH

WASHINGTON (CNN) -- Anticipation of a "contentious" confirmation process on Capitol Hill prompted the decision to replace Gen. Peter Pace as chairman of the Joint Chiefs of Staff when his term ends in September, U.S. Defense Secretary Robert Gates said Friday.

Gates said he will recommend to President Bush that he nominate Adm. Michael G. Mullen, chief of naval operations, to the post.

Pace's renomination to another two-year term had been widely expected, and his nomination papers already had been drafted.

But Gates said some members of Congress warned him that -- despite their respect for Pace -- the focus of any renomination hearing for the general would be on the past, rather than the future, and would be "a very contentious process."

"I am disappointed that circumstances make this kind of a decision necessary. I just think a divisive ordeal at this point is not in the interest of the country or the men and women in uniform or of the individuals," Gates told reporters. "But I think it's a realistic appraisal of where we are." (Watch Gates explain his decision Video)

Gates told Bush two weeks ago that talks with Congress regarding Pace were going poorly, National Security Council spokesman Gordon Johndroe told CNN.

"Last night, Secretary Gates called [national security adviser] Steve Hadley, and Steve Hadley informed the president this morning that Secretary Gates was going to go forward with what had been previously discussed -- that they would not be able to go forward with General Pace's renomination because consultations with the Hill had not gone well," Johndroe said.

Sen. Carl Levin, chairman of the Armed Services Committee, said in a written statement late Friday afternoon that he "solicited the views of a broad range of senators" at the request of Gates.

"I found that the views of many senators reflected my own -- namely, that a confirmation hearing on General Pace's reappointment would have been a backward-looking debate about the last four years," the Michigan Democrat said.

Sen. Hillary Clinton, D-New York, also expected a contentious renomination process, her spokesman said.

"Senator Clinton believes it would have been a difficult renomination, but when it come to Iraq it's not enough for President Bush to change the cast, he must also change their script," spokesman Philippe Reines said in a written statement.

Six years as Joint Chiefs member

Pace has served on the Joint Chiefs of Staff for six years, first as vice chairman and then as chairman. He was the first Marine to hold either post, according to the Department of Defense's Web site. He was sworn in as chairman in September 2005.

The chairman of the Joint Chiefs is the principal military adviser to the president, the defense secretary, the National Security Council and the Homeland Security Council.

Bush, traveling in Poland, issued a statement praising Pace.

"As chairman of the Joint Chiefs, Pete's job has been to help ensure that America's military forces are prepared to meet the threats of this new century," Bush said. "This is a difficult task in a time of peace. Pete Pace has done it in a time of war -- and he has done it superbly.

"As part of our senior military leadership, Pete has helped oversee the liberation of more than 50 million people," Bush said. "I have relied on his unvarnished military judgment and I value his candor, his integrity and his friendship."

Bush also praised the selection of Mullen, saying, "When he is confirmed by the United States Senate, he will make a superb chairman of the Joint Chiefs of Staff."

Asked whether Pace's leaving was related to his conduct of the war in Iraq, Gates replied, "It has absolutely nothing to do with my view of Pace's performance." Gates said Pace was involved in consultations about a likely successor.

Mullen called "very smart, strategic thinker"

Mullen has been chief of naval operations since July 2005. Before then, he was commander of the NATO Joint Force Command Naples, Italy, and commander of U.S. Naval Forces Europe, according to the Navy's Web site. (Read more about Mullen)

Gates described him as "a very smart, strategic thinker" who understands the needs of all branches of the military.

He also has served in leadership positions at the Naval Academy, in the Navy's Bureau of Personnel, in the office of the secretary of defense and on the Navy Staff. He was vice chief of naval operations from August 2003 to October 2004.

"I think, going in, that Admiral Mullen is well-qualified," Levin said.

Gates praised Pace's years of service, saying, "I have thoroughly enjoyed working with him, trust him completely and value his candor and his willingness to speak his mind. I look forward to continuing to work with him until the fall and to a continuing friendship after his retirement."

Vice chairman also retiring

Gates also announced that the vice chairman of the Joint Chiefs, Adm. Edmund Giambastiani, is retiring. Gates said he had offered another senior assignment to Giambastiani, but he chose to retire.

The defense secretary said he will recommend Marine Gen. James Cartwright, now commander of U.S. Strategic Command, to replace Giambastiani.

Bush, in his statement, also thanked Giambastiani, saying he "has earned the thanks of a grateful nation," and said that Cartwright is "an exceptional officer" who will be an outstanding vice chairman.

Sen. John Warner of Virginia, the senior Republican on the Armed Services Committee, gave Gates' decisions a thumbs-up.

"As we look to the future, in Adm. Mullen, we will have a new hand on the helm, a steady, well-trained hand that will guide and protect the men and women of all of our services, and their families," he said.

"Gen. Cartwright has an extraordinary grasp and understanding of the global posture that America must maintain in this era of new and ever-changing threats," he added.

Presidential hopeful Sen. John McCain, campaigning in Iowa, said he strongly supports Gates' choice. "Adm. Mullen ... I think will make a fine chairman of the Joint Chiefs," he said.

But Senate Majority Leader Harry Reid indicated that the confirmation hearings might still have some contentious moments.

"Senators will be looking very closely at Adm. Mullen and Gen. Cartwright's views to make sure they are committed to changing course in Iraq," he said. "Both men must be advocates for our troops, not for a failed policy."

CNN's Barbara Starr and Suzanne Malveaux contributed to this report.