Specialists in the control of strategic nuclear arms are dealing these days in a world of what-ifs: What if the process of negotiated arms control, already on its most uncertain ground in more than a decade, breaks down completely? What if such a breakdown leads to a no-holds-barred arms race?

The questions are admittedly speculative, but experts inside and outside the Reagan administration are grappling with them because the answers are central to the future of international stability in a nuclear age.

What a number of those experts believe is that a decade from now -- after an extraordinary outlay of money by both the United States and the Soviet Union -- the rough strategic balance that exists today between the two countries may well be unchanged.

Paradoxically, while the Reagan administration feels it is the United States that needs to catch up with Moscow, it is the Soviets, with their bigger missiles and open production lines, that are in a better position to set the initial pace in an all-out arms race.

But although its defense industry has dwindled, the United States is not without resources in this contest. With its superior technology and stronger economy -- and given the effort that the administration has pledged to rebuild the nation's defenses -- the United States probably could offset that Soviet spurt and even regain some of the edge this nation once held.

Even so, a number of specialists conclude that in the long run neither superpower, no matter what the cost, will allow the other a clear edge in intercontenental strategic striking power, the nuclear power represented in land- and sea-based missiles and bombers.

It is this prospect that may eventually bring two countries already armed to the teeth back to the negotiating table.

"Neither side will allow superiority, so arms control ultimately is essential," says Seymour L. Zeiberg, deputy undersecretary of defense for strategic systems. "Otherwise we'll drive each other to the poorhouse. There is no benefit to parity at increased levels."

For the Soviets, an uncontrolled arms escalation could mean severe economic problems. For the United States, says Zeiberg, "we will run out of public support . . . and into constant problems with Congress . . . if our only policy is laissez-faire" -- to let things just go on. "You've got to have the goal of eventual reductions" in atomic arms.

But the arms race and the future of talks to limit strategic weapons have arrived at a crucial point. Soviet troops are in Afghanistan and around Poland, and the Soviet missile force appears more menacing than it did a few years ago. In Washington and the rest of the Western world generally, the Soviets are now widely perceived as a more dangerous mischief-maker.

And the watching eyes in Washington are those of a new administration, one that is harshly critical of the Soviets, skeptical of past SALT agreements and convinced that U.S. defenses must be repaired, with or without arms control, so that even today's rough balance of power is not lost.

The difference in attitude toward arms control between the Reagan and Carter administrations is clear. In a parody of the well-worn phrase about the "mad momentum of the arms race," some senior State officials now talk about "the mad momentum of arms control," a phrase coined by University of Chicago professor Albert Wohlstetter.

Reagan administration officials -- among them Richard Burt, director of the State Department's bureau of politico-military affairs -- are known to argue that for a decade the United States has tended to see arms control negotiations as a substitute for making needed improvements in America's arsenal.

In this view, the SALT agreements worked out by the Nixon, Ford and Carter administration, while well-meaning, in effect failed to solve some real problems -- such as the growing vulnerability of the 1,000 U.S. land-based Minuteman missiles to attack by increasingly accurate Soviet missiles. Before the United States can enter into arms control talks, the arguement goes, it must improve the effectiveness of its own forces and the ability of those forces to survive a first attack.

Burt is the head of an interagency group -- comprising officials from the State Department, the Pentagon, the Joint Chiefs of Staff, the Arms Control and Disarmament Agency, the CIA and the White House National Security Council staff -- that has been studying such questions since February.

The group is trying to decide what the United States needs to do to bolster its defenses and then see whether those improvements can be made under the SALT I and antiballistic-missile (ABM) defense treaties signed with Moscow in 1972, as well as under certain provisions of the still-unratified SALT II agreement signed by President Carter and Soviet President Leonid Brezhnev in 1979.

The administration has made it clear that it doesn't like SALT II, which it thinks allows Moscow too much of an edge in the number and size of missiles, but it has been careful not to throw out the whole treaty yet. "We don't want to be stampeded," said a member of Burt's group.

The United States, under pressure from NATO allies to keep some form of arms control alive, is expected to move ahead on negotiations with Moscow on limiting medium-range nuclear missiles based in Europe.

But SALT II, which deals with the big ocean-spanning missiles and bombers, is likely to be abandoned as it now exists, and the administration has not said what, if anything, it will propose in its place.

If the administration does make new SALT proposals to Moscow, defense officials say they expect those proposals to involve sizable reductions in the missile arsenals of both sides. If that doesn't work out, adds a White House official, "equilibrium in a world without SALT will come from competition rather than negotiation and will be established by how fast we can discourage the Soviets from building more land-based ICBMs" of the type that most threaten the United States' deterrent force -- its land-based Minutemen missiles.

It is the fear that the U.S. Minuteman missiles are becoming vulnerable to surprise attach that cools the ardor for arms control and fuels the drive for new, less vulnerable weapons. Of such fear was born the Air Force's proposed MX missile plan, where for a price of $35 billion to $60 billion, the United States could build 200 highly accurate, multiple-warhead missiles and shuttle them among 4,600 underground shelters to confuse Soviet attackers.

The debate between the advocates of arms control and the advocates of new deterrent weaponry is centered on the reasonableness of that fear.

Many arms control advocates acknowledge that the threat to the Minuteman force is at least theoretically true, but they argue that is is on the extreme edge of plausibility. For the Soviets to have even mild confidence that such an attack would work, they would have to launch 2,000 warheads, two for each Minuteman silo, with split-second timing over a flight path never before used. They would be betting that a U.S. president would not, in return, fire everything left in the U.S. arsenal, from submarines, bombers and surviving land-based missiles.

Retired Gen. Maxwell Taylor, the former chairman of the Joint Chiefs of Staff, and Harvard professor Stanley Hoffmann are among many respected observers who have said publicly that what Hoffman called this "nightmare scenario" is not convincing.

Hoffman, on the "Bill Moyers Journal" television show, argued recently that the United States has become "too masochistic about the military balance," and wondered aloud if the only purpose of the MX or a reopened ABM system was "to reassure ourselves" rather than to combat a real threat.

Nevertheless, this administration is concerned about Minutemen vulnerability, as the Carter administration was late in its term. This was partly because of a belief that even the perception of Soviet superiority would be politically useful and partly because American nuclear strategy has been changing in important ways.

The strategy now, as reiterated by the Reagan administration, involves not only seeking to deter war but to "prevail" if one starts. The way to do that with the highest confidence of success, according to the strategy, is to have missile forces that can ride out a nuclear attack and still be able to hit, with pinpoint accuracy, military targets in a tit-for-tat response.

The administration believes that Moscow ultimately is deterred only by the prospect of no military victory. Thus the idea is to convince the Kremlin that they cannot win -- and may lose.

The level of support for this strategy, which goes back four administrations to the work of former Nixon-era defense secretary James R. Schlesinger, should not be underestimated, officials say. In 1979, it led to a series of Carter directives on strategy, on survival of key political and government leaders and on a national telecommunications network that ensures that the president or his successor will always be able to get orders to the missile forces.

It is continuing within the Reagan administration's Office of Management and Budget, where a study of wartime command and control needs is under way.

While this strategy may make good sense to defense planners and some others, they acknowledge that it also seems to ratify, in the public mind, the idea that limited nuclear war is manageable. That is a thesis that scares many people, including U.S. allies on whose territory a limited war might be fought.

Reflecting on the history of superpower arms control efforts, a member of the Reagan administration's interagency study group thinks that the advocates of arms control, by building public expectations too high, have contributed to the current mood of frustration with the process.

Hoffman views it differently: "I think it is just at the moment when arms control, after many illusions and excesses of belief, was beginning to become quite useful . . . including SALT II . . . that we now throw out everything and say 'since it hasn't brought the millenium, it's really no good at all.'"