Just as President Reagan is girding for fights with Congress over defense spending, arms for El Salvador and the nuclear freeze, and for crucial decisions on arms control and the MX missile, his administration is reeling once again from controversy over aides he picked to carry out his policies.

Within recent weeks:

The Republican-controlled Senate Foreign Relations Committee declined to recommend Senate confirmation of Reagan's nomination of Kenneth L. Adelman as director of the Arms Control and Disarmament Agency (ACDA) and passed Adelman's name to the Senate with an unfavorable recommendation.

A memo from the office of the chief U.S. negotiator at strategic nuclear arms talks with the Soviets, retired general Edward L. Rowny, surfaced in the press. It touched off a furor in Congress, the State Department, the arms control agency and the negotiating team because it reportedly impugns the reputations of many officials on the team and in ACDA and calls for replacement of some of them.

New reports appeared questioning the stock market dealings of Thomas C. Reed, a deputy national security assistant to the president and vice chairman of the presidential commission studying what to do about the new MX missile. Within days, it was announced that Reed would leave his White House job after his work on the MX commission ends in April.

Several lawmakers have expressed annoyance at other administration officials on grounds they have had trouble getting answers on aid to El Salvador and on how much money the administration really wants.

Although these controversies center in part on personalities, they are seen by many lawmakers and career officials as nevertheless hurting the credibility of the administration at home and abroad and sapping the ability of the White House to deal effectively with issues.

"It's very debilitating," said one top Republican senator who is a strong Reagan supporter but who, like many of his colleagues, is frustrated by the steady stream of embarrassments and confusion. "I would have to say these developments are certainly unhelpful at a critical time," added Sen. John W. Warner (R-Va.).

"Certainly they do damage," said Sen. Henry M. Jackson (D-Wash.). "There is no doubt about it."

Jackson said, "it's a management problem" and that a big part of it is that Secretary of State George P. Shultz, whom Jackson calls "extremely competent," is "totally overworked" and is called upon too often to address economic issues rather than focusing on foreign policy.

"Clearly, they need to do some rebuilding in the management of national security policy," Jackson said, "and the president has got to be more directly involved" in that management.

Jackson and a number of other lawmakers said that one of the "most glaring" frustrations has been over the profusion of estimates that emerged when Congress was trying to find out how much was wanted for El Salvador.

Sen. William S. Cohen (R-Maine) warned that allied leaders in Europe "see capriciousness in U.S. foreign policy and thus are less able to persuade their constituents of our reliability."

Without criticizing Adelman, Cohen said Adelman's situation "becomes a symbol to those who think we are not committed to arms control." Now, he added, allies see the House of Representatives apparently about to approve a nuclear freeze resolution opposed by the White House. Yet that freeze movement, he said, is "a direct consequence of people feeling frustrated and fearful that this country is not sincere . . . and that's the result of a lack of policy on the part of the administration . . . and its personnel."

Within the arms control and weapons policy areas, the administration faces two new situations as a result of the recent episodes.

State Department and ACDA officials said morale within the U.S. delegation to the Strategic Arms Reduction Talks (START) has been destroyed by the Rowny memo. Although Rowny issued a statement disowning the memo and stating that it did not reflect his views, many officials and lawmakers have no doubts that the views are Rowny's. Sources said the Soviet delegation at Geneva already has begun using the memo against its U.S. counterparts in corridor conversation, suggesting that it shows the Reagan administration was never serious about wanting an agreement.

The Reed case is different. Reagan badly wants the MX missile and officials said that Reed was extremely effective in working out a weapons compromise. Now some say they think his effectiveness has been jeopardized, just as it is most needed. But whether that will affect the MX recommendation remains to be seen.