Deputy Defense Secretary John Hamre is giving up his job as the Pentagon's second highest-ranking civilian to run one of Washington's biggest think tanks, marking the surprise early exit of one of President Clinton's top national security advisers.

Announcing the move yesterday, Defense Secretary William S. Cohen said another senior Pentagon official, Rudy de Leon, would be nominated to fill the deputy's post.

Hamre's departure robs the Pentagon of one of its most experienced managers and skillful political tacticians in the administration's final year.

In his 2 1/2-year tenure as deputy secretary, the popular one-time divinity school student and former Senate aide has been most strongly identified with Pentagon initiatives to improve business practices, guard military computers against attack and improve U.S. defenses against chemical and biological warfare. But Hamre also has influenced a broad range of other budgetary, personnel and policy matters and worked closely with Cohen, a former Republican senator, in managing tensions between the Democratic administration and GOP-controlled Congress.

Explaining his decision to leave at the end of March, Hamre, 49, said he regarded the offer to become president of the Center for Strategic and International Studies (CSIS) as a "once-in-a-lifetime opportunity." Of particular appeal, he said, was the center's centrist reputation and wide coverage of national security subjects.

News that Hamre was leaving the Pentagon drew expressions of regret from many inside and outside the Defense Department.

"The question for me is, who's going to run the Pentagon now?" said Loren Thompson, a director of the Lexington Institute, which specializes in defense issues. "Hamre has been very well-liked, and I don't know if there's anyone who has his range of skills. His departure is one signal among many that we're nearing the end of an administration."

Cohen also lamented the loss of his top lieutenant. "John Hamre has been my friend and partner in running the department," the secretary said in a written statement. "He approaches all issues with extreme confidence and an extraordinary sense of humor."

CSIS, which was founded nearly 40 years ago, ranks among the nation's most prominent public policy research institutions, with a budget of more than $17 million and staff of 90 policy experts and 80 support staff members.

In joining the center, Hamre will reunite with Sam Nunn, the former Democratic senator from Georgia who was chairman of the Armed Services Committee when Hamre worked on the committee's staff. Nunn, now chairman of CSIS's board of trustees, first approached Hamre two months ago about becoming president of the center.

"The center has all the qualities of what I would like to do," Hamre said. "It's nonpartisan, it tries to be actively involved in a consensus way to solve real problems."

Asked whether he had considered other, potentially more lucrative job prospects, Hamre quipped: "You know, the fear I have in life is prosperous boredom."

Senior aides to Cohen expressed confidence that de Leon--who, like Hamre, is a former congressional staff member with strong ties to Capitol Hill and considerable trouble-shooting experience in the Defense Department--would bring to the Pentagon's number two job much of the same political acumen and managerial talent.

According to one well-placed defense official, de Leon emerged early as "the consensus candidate" to succeed Hamre.

"He has Cohen's confidence," the official said. "He also has good ties with the White House and is seen as fair and objective in working with Capitol Hill. He's methodical and cautious and solicitous of people's views."

De Leon, 47, came to the Pentagon at the start of the Clinton administration in 1993 as chief of staff to former secretary Les Aspin. A year later, after Aspin's departure, de Leon became undersecretary of the Air Force.

In 1997, de Leon took over as the department's undersecretary for personnel and readiness--a job that has made him Cohen's point man for some of the Pentagon's toughest, most controversial matters.

He has had responsibility for overseeing implementation of the "don't ask, don't tell" policy on homosexuals in the ranks and for dealing with downward trends in military recruiting, retention and other readiness indicators. Lately, he has supervised initiatives to increase housing and medical benefits for troops.

While seemingly outside his normal portfolio of personnel issues, de Leon also has been deeply involved in the so far unsuccessful negotiations with Puerto Rico to regain Navy use of the Vieques training range.

CAPTION: Deputy Secretary John Hamre will leave Defense to become president of the Center for Strategic and International Studies, which he called a "once-in-a-lifetime opportunity."

CAPTION: Rudy de Leon, the Defense Department's undersecretary for personnel and readiness, is in line to succeed Hamre.