In an election year in which the status of U.S nuclear strength has come in for particularly heavy scrutiny, the gloom is coming from all sides.

Secretary of Defense Harold Brown tells us that the 1,000 U.S. land-based Minuteman intercontinental ballistic missiles (ICBMs) may already be vulnerable to being wiped out in a first strike by increasingly numerous and accurate Soviet missiles.

The shadow defense specialists of the Republican challenger speak of a "window of vulnerability," a dangerous period of several years in the 1980s in which this threat will exist and in which the United States may be vulnerable to political intimidation or worse from Moscow.

It is not surprising, therefore, that opinion polls show many people these days with a sense of insecurity about the nation's military capabilities and its strategic nuclear power.

These are legitimate concerns. Soviet military strength should not be minimized. Yet there are other sides to these issues which are not getting much attention in campaign rhetoric but which are more reassuring to the public. And there are underlying issues, which may have a much more profound impact ultimately on whether the nuclear superpowers can go on keeping the lid on nuclear war.

There are three key questions. Can the United States still deter a Soviet first strike to begin with? Is the Minutemen really vulnerable? If it is, is the administration's proposed $35 billion-$60 billion mobile MX missile the right solution?

In the Pentagon, the answer to the first question is an overwhelming yes, at least for a while. It is the future they are unsure about.

Aside from the Minuteman missiles, the United States still has 656 missiles on 41 submarines -- half of which are always submerged at sea -- and 376 long-range B52 and FB111 bombers, at least 100 of which are always on alert and could get into the air before enemy missiles landed. Thus the United States still has massive atomic firepower, which is widely held as still invulnerable to being destroyed in a surprise attack, and which could respond with devastating retaliation against the Soviets.

Furthermore, new weapons programs already under way tend to get overlooked in alarmist arithmetic. While the new MX, if it is actually deployed, won't enter the arsenal in large numbers until the late 1980s, the United States is already beginning to produce thousands of new air-launched cruise missiles that will begin entering service abroad B52s in 1983. These cruise missiles, if they perform as advertised, can deliver atomic bombs with pinpoint accuracy to Soviet military targets.At about the same time, the United States will begin installing hundreds of new ground-launched cruise missiles in Western Europe.

Yet, the growth of the Soviet missile force, the turmoil in many areas of the world and in Soviet-American relations has produced somewhat of a new situation and with it a new look at deterrence.

Few, if any, specialists expect a so-called "bolt from the blue" surprise attack on Minuteman that would catch the United States flat-footed. For the Soviets to launch such an attack they would have to have put many more of their ICBMs on alert and get many more of their missile submarines to sea than is normal and this activity is likely to be detected by various U.S. warning systems.

Rather, the greater risk is seen as growing out of a long and deep crisis for Moscow in which political or military events suggest to the Kremlin that things are coming apart for them. Striking first might then seem the lesser of two evils. It could be the difference, as Deputy Assistant Secretary of Defense Paul D. Wolfowitz recently put it, "between a Pyrrhic victory and a Pyrrhic defeat" for the Soviets.

In this view, the Soviets, having lost 20 million people in World War II, understand better than Americans that societies, despite extraordinary suffering, can survive and rebuild and that surviving military forces are important.

Taking out Minuteman in a first strike, especially the 550 Minuteman III versions that each carry these warheads and are the most accurate U.S. missiles, would confront an American president with a gruesome choice, the argument goes. Would he retaliate with the less accurate U.S. submarine-based missiles and bombers against Soviet cities and industry, knowing the Soviets still had many weapons left to then knock out U.S. cities in a mutually suicidal exchange? Or would he yield to the results of that first Soviet attack or even to the threat of attack?

That leads to the question of whether Minuteman is really vulnerable.

Even many leading critics of Pentagon assessments say that the land-based U.S. ICBMs will at some point become at least theoretically vulnerable to the Soviet missiles. Within government, the "worst case" estimate is that only 5 percent -- 50 missiles -- would survive an attack in which the Soviets lobbed two warheads at each Minuteman underground silo. But not many people credit the Soviets with the ability to carry out a near-perfect strike, so that the most likely current estimates are that 100 to 300 missiles might survive, which could represent a sizable counterpunch in Moscow's eyes.

The Minuteman is most important to Moscow because it is currently the only missile with enough accuracy to at least threaten to knock out a portion of the Soviet missile force in its silos.

In fact, however, it may be that even larger numbers of Minuteman would survive. For one thing, there is simply no experience in launching hundreds of missiles in a coordinated attack within seconds of each other and expecting that all but a few would work. Aiming two warheads at a single target could cause one to be blown off course or destroyed by the blast from the first one unless the timing was perfect.

Missile accuracies, at best, are estimates. Indeed, some experts, such as Richard Garwin of the IBM Research Center, have warned that there are additional "biases" -- the difference between the calculated center of missile impact points and the actual target location -- that remove the confidence of any planner in a successful strike, especially one on which the future of his country is riding. In war, missiles would fly over paths completely different from those flown in tests, with different winds and geophysical conditions pulling or pushing on the missiles, also affecting accuracy.

Most importantly, however, it would take 1,000 Soviet atomic bombs exploding in the northwestern United States to carry out a strike just on the 550 Minuteman IIIs. It would kill two million to 20 million Americans, government sources estimate. Is it possible that an American president would accept that without either firing before those missiles landed or letting the whole arsenal go in retaliation? Certainly the Soviet leaders could not count on that with any confidence.

Thus, the question of Minuteman vulnerability is on the very extreme of threats to this country.

Indeed, such emphasis on this nuclear question during this campaign year may turn out to be a serious, selfinflicted wound. In the view of many defense analysts, it is the state of the country's conventional forces -- the Army divisions, Navy ships and Air Force planes -- that should be of much greater concern since they are far more likely to be the ones used in future confrontations.

"The MX is not the answer to the Persian Gulf," is the way former State Department intelligence chief William Hyland recently put it.

If the Minuteman force is vulnerable, it raises questions about the wisdom of putting new billions into new land-based missiles such as MX, rather than just leaving the existing Minutemen where they are -- since Moscow would still have to be worried about them -- and placing more of our future strategic armament in less vulnerable submarines and bombers.

Defenders of land-based missiles argue that they are vital because -- in contrast to submerged submarines or airborne bombers -- their crews are the easiest for a president to communicate with quickly and confidently in a crisis. Missiles launched from land are also generally more accurate than those launched from sea or air. Significantly, even many government skeptics believe it is important that the United States retain its three-legged weapons policy of land-, sea- and air-based weaponry to protect against a Soviet breakthrough that could nullify one or even two of those forces.

The Carter administration contends that MX is the answer because the 200 proposed new missiles will be frequently moved among 4,600 shelter in an extraordinary version of the shell game that will keep the Russians from knowing where the missiles are at any time.

But MX also could introduce a hairtrigger aspect to nuclear war that could make things even more unstable. This is because these missiles, each of which will carry 10 powerful and highly accurate individual warheads, will give the United States for the first time a real possibility of knocking out most if not all of the entire Soviet land-based force of 1,400 missiles in a first strike. The Soviets, unlike the United States, have 75 percent of their striking power in land-based missiles, so their loss would be catastrophic.

The adminstration argues that the MX is needed precisely because it will be invulnerable and therefore cold ride out a Soviet first strike and thus deter Moscow from launching such an attack. But would Moscow see it that way? Probably not. Thus, the temptation for the Soviets to launch first in the midst of a severe crisis may be even greater.

The MX, therefore, is not simply a means of countering something the Russians are already putting in place. It means that old notions of nuclear stability are ending with no clear idea of where the next point of stability is.

It is a topic worthy not only of campaign debate but also of the kind of discussion and vote that Congress conducted a decade ago when the country faced another strategic decision of extraordinary consequence -- the debate over anti-missile defenses.