Ronald Reagan's senior defense adviser says that a Reagan administration would seriously consider digging thousands of new underground silos as a way of disguising the locations of existing U.S. Minuteman missiles and, for the short run at least, reducing their vulnerability to a disabling Soviet first strike.

This idea of moving rapidly to reduce the alleged vulnerability of the U.S. missile force emerged as one of the major contrasts between Reagan and President Carter in a lengthy interview with Reagan adviser William Van Cleave, an effort to pin down specific differences between the two presidential contenders.

There are other contrasts, both in style and substance. But what also emerged from the wide-ranging session is that the Reagan forces are not yet able to say exactly what they would do differently on some key issues, while the Carter administration, which recently has put more emphasis on defense, is moving toward some of the same solutions Reagan would likely propose.

Van Cleave says he "fully expects" to be part of a Reagan administration if Reagan wins next month. Thus his views are important, and his generally harder line toward Moscow, tougher than is evident in the Defense Department rhetoric under the current administration, could also be a major change.

Both camps have claimed that the 1,000 U.S. Minuteman land-based missiles are becoming vulnerable to a Soviet attack. The Carter administration's answer is to move ahead with the Air Force's massive new MX mobile missile project. But these missiles would not begin entering service until 1986 at the earliest.

Van Cleave says his goal would be to resolve the vulnerability problem sooner, in the belief that the nuclear balance between the two superpowers is "the high ground" and once it has been lost, U.S. military credibility is weakened across the board.

So he is studying applying to the Minuteman the controversial "shell game" principle of the MX, in which 200 MX missiles would be shuttled amoung 4,600 shelters.

Van Cleave and his staff are studying the possibility of building 5,000 or more additional silos for the existing Minutemen, plus canisters for hauling the missiles around, as a way to make it more difficult, if not impossible, for the Soviets to know where they are and attack all the missiles at one time.

If this can be done in 3 1/2 years, an estimate the Air Force believes is optimistic, then Van Cleave feels the United States could cut about three years off that period of Minuteman vulnerability before even the first MX is fielded.

Van Cleave says that what he is studying are "options" and that the new administration would have to get into the Pentagon and its computers and staffs before it could make up its mind. But it is clear that another important difference between the two administrations may be the ultimate disposition of the MX, the biggest military project ever at an estimated cost of between $35 billion and $60 billion as now constituted.

"I think we would certainly go ahead with the development and production of the Mx missile," he said. But a Reagan administration would want to review the controversial and complex system with an eye toward simplifying and speeding up the project, Van Cleave said. He doesn't favor putting MX missiles on submarines, as some have suggested.

Another option to which "we have to give serious consideration," he adds, is that of defending U.S. missiles against attack with an antiballistic missile (ABM) defense system. Because a U.S.-Soviet treaty limits such weapons, Van Cleave says, "we'd have to try and get that renegotiated" if ABM defense looks like a good option.

This represents another major potential departure from Carter administration planning, though the Pentagon has always continued research work on ABMs.

Van Cleave's concern about the vulnerablility of U.S. nuclear retaliatory forces extends to the fleet of B52 bombers. He feels that too few are on alert and that many bombers should be moved away from bases on the coast because they are too vulnerable to attack by submarine-launched missiles.

He also says major changes must be made in the "extraordinarily fragile" command and control system that the president would use to order U.S. nuclear forces into battle after an attack. The vulnerability of these links also has been recognized by the administration.

Van Cleave's name is hardly a household word, but the 45-year-old director of the strategic studies program at the University of Southern California is well known in defense circles. At times, he also has been at the center of controversy.

In 1971, toward the close of a two-year Pentagon stint as a special assistant to the secretary of defense, Van Cleave was suspected of leaking information about the U.S.-Soviet SALT negotiations to The New York Times. Van Cleave says he made no secret of his unhappiness over how the negotiations were going, but he denied he was the source of the leaks, which were potentially damaging to the arms talks.

Van Cleave recalls that the "good old plumbers" of the Nixon administration had focused on him as the leak. To clear himself, he agreed to take a lie detector test, but only if administered by the FBI rather than the "plumbers." He says he took three succh tests and "came out absolutely clear."

In 1968, Van Cleave coauthored an article in the National Review entitled "Assertive Disarmament," which dealt with options for the United States, alone or with other nations, to neutralize or even destroy China's fledgling atomic weapons production capability by various means, including infiltraiton of military teams. The article concluded that, until assertive disarmament is proved unacceptable or infeasible, the United States should be prepared to invoke the option, if necessary.

Some of Van Cleave's critics these days, who worry about where his hardline, anti-Soviet view will lead, feel this earlier article reveals a touch of recklessness.

Van Cleave says the article was written in the days of China's unpredictable "Red Guards," when there was great concern in arms control circles about China and the bomb. The article, he says, was meant to examine all the options and pros and cons. It "reached no conclusions whatsoever," he said. "We didn't advocate doing it, or anything."

Van Cleave makes no secret of his "fairly somber view of Soviet objectives" in the world. As he sizes up the national security risks facing the United States in the decade ahead, he sees the Soviets as "the perfect 10." The Soviets are not the only source of problems, he says, but in comparison to Moscow's actions, all the other risks "tend to be 5s and below."

The Soviet Union is clearly a politically totalitarian regime and ideologically militant," one that has "a history of trying to exacerbate and take advantage of" U.S. or regional problems. But, he adds, "I believe them also to be highly realistic in international affairs."

"It is really a question of opportunities opened or foreclosed. I believe that if you can persuade them that the opportunities for extending their hegemony are limited and the attempt to do so would be costly and uncertain, then you're more likely to lead them to the view that it's in their interest to be more cooperative," he says.

To be able to "persuade" Moscow, Van Cleave says the United States must have a stronger defense posture and a bigger defense budget, must move more quickly to patch up existing gaps and must be more consistent in maintaining its defenses.

In assessing the differences between Carter and Reagan on defense he says, "a lot depends on how much you really believe this administration in terms of their intent to carry through programs that they now have on paper . . . on the reality of their declaratory objectives. If you believe them very strongly, then it may appear that there's going to be not that great a difference between the two."

"We also criticize this administration," he says, "for always pointing to something that is just ahead -- whether it's 10 years ahead like a Stealth bomber, or the MX program. But it doesn't handle the problems we are facing right away," such as the nuclear vulnerability question and the need to improve the quality of armed forces personnel.

Van Cleave fears that the president's pledge to protect Persian Gulf oil is one of those "great gaps between rhetoric and reality." He agrees that the United States has vital interests there and ought to be able to protect them. But he believes the United States does not have the capability to take on that commitment now, and to make it anyway is a dangerous bluff.

On his desk in campaign staff offices here are a stack of loose-leaf notebooks in which Van Cleave is compiling what would be the first Reagan defense budget for fiscal 1982, and an eight-year defense plan.

Van Cleave says it is still too early to put a price tag on that budget, that access to Pentagon statistics on such things as operating costs are necessary first. This omission has added to Reagan's credibility problem in the campaign, because voters have no idea how much all the extra defense he advocates would cost or where the money would come from. But the interview reveals that another reason the budget figure isn't clear is that many key decisions have not been made.

For example, does the United States, because of its new commitments, need a bigger Army and Navy? Van Cleave says the country certainly needs a larger fleet and more sailors to man it. He said a Reagan administration would build more ships than are currently planned but hasn't settled on a number yet, though some advisers use a 600-ship fleet as a goal.

The Carter administration's goal, according to Defense Secretary Harold Brown, is to move from 540 ships today to 590 by 1984.

Van Cleave says some of his staff would like to see three divisions added to the Army. Manpower is the most expensive addition to the military. But Van Cleave says the increase need not be big, maybe 10 to 15 percent in the overall size of the armed forces. Any added divisions might be smaller to improve the teeth-to-tail ratio of combat-to-supply troops. But "it's hard to say right now what we'd pursue," he says.

Could we get the additional manpower without a draft? "Quite frankly, there's a division in the advice and recommendations that I'm getting on that," he says. "Gov. Reagan is philosophically opposed to peacetime conscription unless there's a clear need for it."

Reagan and some of the defense staff believe the all-volunteer army, if properly paid, can supply both the quality and quantity needed. "Other advisers don't believe that . . . but we're committed to giving the all-volunteer force a chance to succeed," he said. That is also the Carter administration's position.

To beef up the U.S. presence in the Indian Ocean region, the Carter administration has made arrangements for use of facilities in three countries. Van Cleave thinks these actions are a step in the right direction. But he says there were danger signs in the region many years ago, that previous administrations warned about it and that the current administration wasted two years flirting with unrealistic proposals for demilitarizing the Indian Ocean.

Van Cleave says he'd like to see further arrangements with Egypt and Israel for U.S. access to at least one and possibly both of the airbases in the Sinai desert that were recently transferred from Israel to Egypt with U.S. assistance. Van Cleave also wants to put together a solid front of moderate Arab countries, plus the United States, Israel and Egypt, to reflect a regional realization that Moscow is the main threat to all of them.

Van Cleave says a Reagan administration would not likely make shifts in the traditional deployment of U.S. forces in Europe and Asia. He said he would try to boost U.S. air and naval power as the critical forces in the Middle East and would shy away from use of American ground forces in the region.

Would a Reagan administration be inclined to grant Saudi Arabia the additional equipment it wants to give its U.S.-built F15 fighters added punch? Van Cleave says "that's a tough one." If the Saudis were more forthcoming politically in terms of their relationship with Israel, in their willingness to cooperate with the United States as a force for stability in the region and in helping the United States build a strategic oil reserve, "then I'd be much more inclined to be more forthcoming on military aid and equipment."

"But giving expanded capabilities like that when there's no progress in the political area, I just think that's a bad idea," he said.