Defense Secretary Robert M. Gates said Friday he will ask President Bush to replace Marine Gen. Peter Pace as chairman of the Joint Chiefs of Staff rather than risk a contentious Senate confirmation battle by seeking a second term for Pace.

The Defense secretary said he is recommending the nomination of Adm. Michael G. Mullen, currently chief of naval operations, to succeed Pace.

“After consultations over the course of several weeks with both Republican and Democratic senators, I concluded that because General Pace has served as chairman and vice chairman of the Joint Chiefs of Staff for the last six years, the focus of his confirmation process would have been on the past rather than the future, and further that there was the very real prospect the process would be quite contentious,” Gates said.

Pace, who has served in the top post since Oct. 1, 2005, will retire when his term expires Sept. 30. Confirmation hearings for Mullen are not likely until September.

Gates said he would recommend the nomination of Gen. James E. Cartwright, currently the commander of Strategic Command, to be the next vice chairman of the Joint Chiefs, succeeding Adm. Edmund G. Giambastiani Jr., who also will to retire.

Key members of Congress told Gates Pace might not be confirmed to a second term and his nomination would trigger a divisive confirmation process focused on his role in the Iraq and Afghanistan conflicts.