A kind of shadow government advocating major changes in the United States defense budget is trying to influence the Reagan administration and Congress. Made up of private defense analysts, out-of-power Democratic experts, and growing numbers of business executives worried about the federal deficit, this group says that accelerated weapons procurement is actually undermining national strength. It warns that this is the result of proportionately lower amounts being spent on operations, maintenance, and other budget items crucial to military readiness and combat sustainability.
In congressional testimony, the Joint Chiefs of Staff chairman, Gen. John W. Vessey Jr., has been insisting that ''the proper balance has been struck . . . between providing for present needs in readiness and sustainability and ensuring future capabilities through modernization.''
But General Vessey and other members of the joint chiefs called a special briefing for reporters Tuesday afternoon to rebut persistent questions about the way the armed services gauge readiness. There are also reports that combat units may not be as ready to fight as the Pentagon says they are.
Under President Reagan, spending for military investment (weapons procurement , research and develoment of new weapons, and military construction) has been increasing more than twice as fast as the funds needed to operate and maintain these new weapon systems, according to Pentagon budget documents.
At the same time, that portion of national defense spending which is essentially uncontrollable (money already authorized for future weapons growth plus military retirement pay) has grown to 43 percent of defense spending and will continue to climb past 50 percent during the next five years, without significant changes. Also, the Pentagon's backlog of unobligated or unspent funds has more than doubled to nearly $200 billion.
''This blind stress on big-ticket procurement is destroying the balanced defense posture this country needs,'' Democrats for Defense, a group of former senior Pentagon officials, warned in a recent statement. ''Starting so many programs adds to the huge 'bow wave' of procurement, which will further squeeze out funding for adequate readiness in future years.''
One leading expert who has worked on defense budgets for both Republican and Democratic administrations says the answer is to slow down the pace of weapons modernization and eliminate duplicative systems, especially strategic nuclear weapons.
William Kaufmann of MIT, in a report sponsored by the Committee for National Security, estimates that as much as $238 billion could thus be reasonably trimmed from budget authority over the next five years. This includes halting MX missile and B-1 bomber programs, cancelling the Navy's nuclear cruise missiles and planned new aircraft carriers, foregoing chemical weapons modernization, and substituting less complex tactical aircraft for those sought by the Air Force and Navy.
''There is an enormous amount of duplication, both among services and within services,'' Professor Kaufmann said this week.
In a recent letter to members of Congress, Business Executives for National Security (which has some 1,100 members, including former senior military officers) called for a ''clearly articulated defense strategy, with strategic priorities linked to spending priorities.'' The group questions the worthiness of several major administration defense programs including the B-1 bomber, MX missile, and ballistic missile (''Star Wars'') defense.
The administration wants to lower the rate of requested defense increases from 13 percent for 1985 to about 4 percent in the latter part of the 1980s. But some analysts warn that the down payments already made on many new weapons programs make this unlikely.
''Pressures currently being locked into the defense budget are making spending uncontrollable well through the decade,'' according to the Center on Budget and Policy Priorities, a Washington-based research organization.
In general, Congress during the first three years of the Reagan term has not heeded such warnings. Except for chemical weapons, it has approved every new weapon system.
But one major weapon stands out as a likely target for possible cuts this year - the MX missile, which narrowly survived House approval in 1983 by a handful of votes. With arms control talks stalled and every Democratic presidential candidate opposing the MX, many observers expect the MX budget - if not killed - to be cut from 40 missiles to 21, the number approved last year.