A bipartisan group of former Cabinet officials unloaded a broadside yesterday at the pace of the Reagan military buildup and said "there is no reason to believe that throwing money at defense now will achieve more satisfactory results than throwing money at social programs in the 1960s."

Overspending on the military will weaken the national economy and create a political backlash that will weaken the national security as well, the six former Cabinet officers wrote in a letter to President Reagan's national security adviser William P. Clark.

"In the present fiscal climate excessive short-term military spending can actually be harmful by undermining the political consensus required for a sustained defense buildup," said the group, which was put together by Peter G. Peterson, secretary of commerce in the Nixon administration.

"History teaches that no nation can long maintain a strong foreign policy without a strong economy. One need only look at countries whose economies have declined to see how rapidly they have turned inward, become absorbed with protectionist measures, and faded as major forces in the world."

Five former secretaries of the treasury joined Peterson in signing the letter: W. Michael Blumenthal, John B. Connally, C. Douglas Dillon, Henry H. Fowler and William E. Simon.

In January those same six former officials enlisted 500 business leaders and academicians in co-sponsoring full-page newspaper ads that called on the Reagan administration and Congress to cut $25 billion from defense spending in fiscal 1985 and $60 billion from middle-class entitlement programs and to enact $60 billion in tax increases.

The ad, which represented a major break by the nation's business community from its support of Reagan budget policies, provoked Clark to write the six a letter last month defending the military spending.

He said the sharp increasses proposed by the administration were necessary to redress the "chronic underfunding" of defense in the 1970s and added that "the proposed defense budget is not the source of the deficit problem.

"As a percentage of either total federal spending or the GNP, the cost of our current defense program is less than that devoted to defense in the 1950s and the 1960s," Clark wrote.

"It is proportionately large only when compared to underfunding in the decade of the 1970s," Clark declared.

The six responded: "Nostalgic comparisons with earlier periods are not necessarily relevant. Not only was our fiscal situation more balanced in those years, but our economy was performing far better than it is now."

They acknowledged that "none of us is an expert in the field of strategic planning or military procurement," but went on to list several broad areas of military spending where they believed, after consulting with experts, that proposed expenditures had not been fully justified:

"Questionable military missions." The letter suggested that the administration is pursuing three unrealistic and expensive capabilities: to initiate military action in one geographic area in response to Soviet aggression in another; to launch sea-based air attacks on Soviet ports, and to prevail in a nuclear war.

"Duplication and redundancy." Among examples the letter cited were simultaneously improving the B52H penetrating bomber, procuring the B1 penetrating bomber, developing a Stealth penetrating bomber and purchasing air-launched cruise missiles.

"Expenditures with ill-defined purposes." The letter posed several questions: Is it necessary to have troops with ammunition stocks sufficient for 90 days of combat? Should the administration increase military personnel by 10 percent by 1988, or could the manpower increases be more economically achieved in the reserves or National Guard? Could military bases around the world be cut back?