Meeting with a core group of longtime Reagan supporters in the White House last week, deputy chief of staff Michael K. Deaver revealed the inside story of why he has been able to lose 40 pounds during the last year.

"The secret of the Deaver Diet," he quipped, "is that you only eat on days when senior staff members are speaking to each other."

If they follow this rule, neither Deaver nor his colleagues are in danger of perishing from overeating. Even by the standards of an administration with as many rival power centers as the Democrats have presidential candidates, the conflict between the first-floor forces of White House chief of staff James A. Baker III and the basement troops of national security adviser William P. Clark is significant.

Although Baker and Clark have managed a show of unity behind the president's Central America policy, on which they see eye to eye, communication between the two sides is virtually nonexistent. The Baker side holds Clark and Secretary of Defense Caspar W. Weinberger responsible for alienation of the Senate Republican leadership on the defense budget. The Clark side continues to suggest that Baker and presidential assistant Richard G. Darman pursue a separate agenda not necessarily in the best interests of the president. Recent articles in the news magazines and The Washington Post about this battle have widened the rift.

The missing ingredient in this protracted and sometimes petty conflict is the president. One high official maintains that Reagan does not realize the depth of the division because senior advisers muster a show of collegiality in his presence. That is what he wants them to do. Reagan's historic strategy for dealing with staff conflicts, when he acknowledges their existence, has been to trivialize them. "The boys," he has been known to say, "will work it out."

But what is involved this time is more than just another intramural quarrel among "the boys." Since Reagan consciously delegates large amounts of authority to subordinates, as he did in giving Weinberger the final say on defense budget options, White House office politics can result in shaping national policy. And it can also shape the way Reagan is viewed by outsiders who would like the president to show that he can manage his own administration.

Reagan is so far above the battle that some close to him are uncertain where he would come out if he did decide to intervene. His concern about the Soviet military machine, which the United States now proposes to bolster with more wheat, is unrelenting and genuine. This makes him susceptible to arguments that U.S. flexibility is dangerous, a view he shares with Weinberger and Clark. But Reagan also relishes success in his dealings with Congress, a tendency that makes compromise worthwhile and enhances the position of Baker and Darman.

Since the president shuns involvement in day-to-day detail, he probably doesn't realize the paralyzing tendency of the conflict among his two most powerful advisers. On some key issues, there is almost no communication between the first floor and the basement. White House officials dealing with the media or Congress or outside constituencies complain that they often lack basic guidance about what is going on in national security affairs.

Presently, the struggle is being carried on through a shadow war in which each side accuses the other of "leaks," a word defined so broadly by the president that it appears to include any story not the product of an official announcement. The supposed "leaks" are a secondary issue but a primary way of getting Reagan's attention. Predictably, after a story annoys him, the president will briefly join the battle by again instructing aides to keep their opinions about each other to themselves.

Is this good enough? At a time when administration policies are in trouble on Capitol Hill and skepticism about Reagan's leadership appears to be deepening abroad, such edicts will not solve the problem. If the president wants his staff to pull together, he is going to have to involve himself in the process and do something other than tell his advisers to keep their mouths shut. So far, there are no signs that Reagan is willing to become this engaged.

Thomas C. Reed, the embattled White House consultant who supposedly left for California when he finished his work as vice chairman of the President's Commission on Strategic Forces, was back in town last week to chat with Clark and legislative liaison Kenneth M. Duberstein.

Reed has been replaced by Eugene Poe, an Air Force general, as the coordinator of efforts to sell the MX on Capitol Hill. But Reed has retained membership on the Defense Science Board and officials said that he remains "on call" to talk to any congressman about the president's strategic forces proposal.

"The commission's report is of great import to me and I have no intention of abandoning it," said Reed, who added that he will spend most of this week in California making speeches and talking to editorial boards in behalf of the president's proposal. Reaganism of the Week: (At the signing of the Social Security bill on the South Lawn last Wednesday): "And I am now going over and sign, and as you can notice how cold it is, 12 pens there are too cold--they can only sign one letter, each pen. If my name came out to 13 letters, I would have misspelled it."