There is little argument among U.S. and allied defense specialists that the image, if not the reality, of American nuclear striking power has declined while that of the Soviet Union has grown.

"We have had an arms race for the past 15 years," former defense secretary James R. Schlesinger contends, "but one of the nations -- the United States -- has not been competing."

Changing the strategic balance is not something that can be done quickly. New weapons take years -- and billions of dollars -- to produce.

The Reagan administration, intent on changing the nation's defense image, has asked for a huge five-year defense rebuilding plan that would cost about $1.3 trillion. Asking for money is one quick way to at least give the impression of regained strength before new missiles, bombers and submarines reach the field.

The administration, concerned that the U.S. deterrent force of 1,000 land-based Minuteman missiles is becoming at least theoretically vulnerable to a surprise first-strike attack with Soviet missiles, also is trying to make sure that the money it spends will keep the force secure no matter what Moscow does.

Whether that kind of security can be bought is a $100 billion question that, unfortunately for both Moscow and Washington, has no clear answer. Yet, unless some form of strategic arms limitation talks (SALT) between the two superpowers is restored, which now seems doubtful, both nations will have to decide how to buy security from a vast array of expensive choices.

Former defense secretary Harold Brown has estimated that the United States would spend $15 billion a year on strategic arms, even under the still-unratified SALT II agreement negotiated by the Carter administration. sThe Central Intelligence Agency estimates that the Soviets already spend three times as much in this area as the United States does.

So a world without SALT could become the most expensive scramble yet for an elusive and possibly unattainable edge. Yet as Larry Smith, chief aide to Sen. Gary Hart (D-Colo.), put it: "There will be no way to keep score, no criteria" to tell who is ahead in that scramble. "It will be like Ruth versus Cobb; a lot of shouting into a vacuum."

Without the limits of arms control, what can -- and cannot -- each superpower do to improve its strategic position?Here are the outlooks for each country. The Soviet Side

The Soviets have roughly 1,4000 land-based intercontinental-range balistic missiles (ICBMs). THE SALT II agreement, which may be discarded by Reagan, would limit to 820 the number of ICBMs that can carry the accurate and deadly MIRV-style multiple warheads. These MIRVed missiles are the main threat to U.S. underground missile silos. The Soviets now have 6,000 to 7,000 individual warheads on their ICBMs that could be launched toward the United States, administration sources say.

For several years, Moscow has been replacing old ICBMs with newer ones, which is allowed by the first SALT agreement. They could continue doing that for a while beyond SALT levels if all arms controls were dropped. Because some of their missiles are very big, they could also put more warheads on each missile.

U.S. officials estimate the Soviets could increase the number of warheads by as much as 30 percent with "moderate" efforts, although there would be some loss of confidence in the performance of each warhead as more are crammed into a single missile. If the Soviets were to attempt a much greater expansion, they would run into the problem of increasing expense and possibly of being able to produce nuclear material to put into those new warheads, as well as having enough construction industry capability to build additional underground missile silos.

Because existing SALT treaties limit "launchers," meaning silos or submarines, rather than actual missiles, those experts most alarmed by the Soviets also worry about a "breakout," meaning a secret ability on the part of the Soviets to accelerate deployment of ICBMs and defensive antiballisti antiballistic missiles (ABMs) as soon as SALT is thrown out or if war seems imminent.

There is some evidence to support this fear. First, two major Soviet missiles, the Ss17, which carries four warheads, and the big Ss18, which carries 10 individual bombs in its nose, are "cold launch" weapons. This means they are popped out of their silos by air pressure rather than being fired out by their own rockets. This leaves the silo relatively undamaged and thus theoretically available for reloading with other missiles secretly produced and stored.

Second, factories where Soviet missiles are procuced tend to have lots more floor space than is needed to produce the number of missiles detected coming out of them by U.S. spy satellites.

On the other hand, U.S. defense officials say there is "no convincing technical evidence" of large secret stockpiles of Soviet missiles, beyond those that both sides are known to have for tests and spares (the United States has about 130 of the most advanced Minuteman, and smaller numbers of older models, in storage, officials say).

Furthermore, strategists claim U.S. atomic bombs would burst over all major Soviet missile based in any war, making these complexes radioactive for long periods. Thus it is unlikely that crews could come in and reload silos.

Brown has estimated that, without SALT, the Soviets could have 14,000 to 17,000 individual warheads on their ICBM force by 1990. That would be more than enough to overwhelm the planned U.S. deployment of 200 mobile MX missiles shuttling around between 4,600 protective shelters. It would also be close to the number needed to overwhelm even a double-sized MS-shelter scheme that is now being discussed inside the administration. It is generally assumed an attacker would have to aim two warheads at each shelter to be sure of knocking it out.

But a number of specialists doubt that Moscow would or could expand its weapon production that much. These doubts are based partly on Western estimates of severe economic problems for Moscow late in the decade and partly on the view that the Kremlin may not want to put so much dependence on land-based ICBMs because they will become vulnerable to a strike by the future MX force.

Unlike this country, which has only about 40 percent of its nuclear punch on land-based ICBMs rather than on submarines or bombers, the Soviets have about 75 percent of their force on land. The decision of whether or not to engage in an open-ended deployment of land-based missiles is what Moscow would have to face if the United States goes ahead with an ever-expanding MS program, whether the MX is eventually deployed on land or at sea. Some administration officials believe that Moscow may choose to negotiate instead and thus bring the superpowers back to the world of SALT.

Other Soviet specialists, such as former CIA analyst William T. Lee, believe Moscow eventually will turn to an antiballistic missile (ABM) defense system, in the hope of nullifying the MX missiles by shooting them down. Lee believes the Soviets will turn to missile defense -- the last real gap in their overall military strategy -- and that the Soviet threat, therefore, will be even worse late in the decade than it is now.

Carter administration officials argued that ratification of SALT II was in the interest of the United States because the agreement would keep Moscow from adding even more missiles while still allowing all U.S. programs now under development, such as MX, to continue.

The SALT II agreement, however, would have expired in 1985. MX will not even begin to be fielded until 1987, and there is no public record that indicates the Soviets would continue to agree to the SALT limitations once MX is deployed. The U.S. Side

Through the pace and extent of the Soviet buildup has eroded this country's former lead in strategic power, the United States retains a big, potent and diverse force of 1,053 ICBMs, 39 missile-firing submarines with 624 missiles aboard and more than 300 old but useful B52 bombers.

This force cannot be knocked out all at once and it retains, for the foreseeable future, the ability to devastate Soviet society.

U.S. officials say this country could cope with a 25 percent Soviet expansion in warhead numbers by dispersing bombers to inland bases, out of the reach of Soviet missiles fired from submarines, by dispersing more submarines to overseas bases and by keeping on alert more bombers and submarines, both of which are less vulnerable to attack.

Beyond that, the United States could accelerate production of new nuclear-tipped cruise missiles for the B52s. After 1982, when the current U.S.-U.S.S.R. ABM treaty comes up for review, the United States also could go ahead with a missile defense network, an area where American technology is ahead of the Soviets'. A White House official says the new administration has a much more positive attitude toward missile defense -- perhaps as a way to defend the MX force -- than did the Carter administration.

The United States also could, as could the Soviets, keep some old submarines in service longer and take some missiles out of storage.

But despite some fancier schemes that remain under study, the Reagan administration thus far is continuing with the major elements of the U.S. defense plan outlined during the Ford and Carter administrations, including a land- or sea-based MX missile and the Trident missile submarine to replace the older Polaris vessels. The big addition has been a decision to put a new bomber into production.

If any of this brings security, it also carries dangers, at least in the view of some specialists.

Walter Slocombe, a deputy undersecretary of defense in the Carter administration who supports much of the Reagan defense program, believes that going back to ABMs will greatly increase nuclear instability by introducing a false sense of security that counterattacks can be dealt with.

He worries also that superpower emphasis on atomic arms will weaken the case for nuclear nonproliferation around the world and detract attention and money from conventional military forces, which are the ones most likely to be needed.

A former Carter White House official, who asked not to be identified, believes that new weapons like the MX, while they are less vulnerable to Soviet attack, also will put a hair-trigger on the nuclear button because they pose a clear threat to Soviet missiles and thus will make Moscow even more likely to strike first rather than risk having its huge missile force wiped out.

But Seymour L. Zeiberg, a deputy undersecretary of defense for strategic systems, argues that it is only Moscow that poses that first-strike threat now and that things will be more stable -- not less -- when both nations have it.