The MX missile debate is dominated these days by the politically and strategically explosive question of where to base it, while a number of other fundamental questions bearing on the issue tend to be overlooked, at least in public.

For example, is there any chance that the all-but-forgotten role of arms control could provide a potential answer to the threat that Soviet missiles may present to this country and that new MX missiles will present to Moscow?

* Could the Soviets really expand their missile arsenal enough to challenge MX?

* Will antiballistic missile (ABM) defenses, which may be needed to protect MX, really work?

* Will the public accept paying the bill for those ABM defenses, and will the public support the modifying of an existing treaty with Moscow that limits ABMs?

* And finally, how vulnerable to attack are the existing 1,000 U.S. Minuteman land-based missiles?

The official assumption that Soviet missiles are now sufficiently accurate and numerous virtually to wipe out the Minutemen in a first strike is the gear that drives the MX program and raises questions about the future of land-based missiles generally. The administration argues that MX must be made invulnerable to survive and that this country must be able to pose the same threat to the Kremlin's missiles as the Soviets pose to ours.

These issues are not simple matters of being for or against MX or some other new so-called "common" missile that might be used by both the Air Force and Navy instead of MX. They are being used by supporters on all sides of the basic question of what to do about the new weapon. The issues are extremely complex and are probably followed in detail by a relative handful of people outside a similarly small circle making the decisions at the top of government.

Yet the forthcoming decision on the next generation of U.S. land-based missiles could involve more than $100 billion in federal spending over the next several years, especially if an ABM network is added. It will set the course for the U.S. nuclear arsenal well into the next century and have an extraordinary impact on arms control. It will confront Moscow with as tough a decision as the White House is now facing and will perhaps put even more of a hair trigger on the nuclear button.

The double-edge on these questions is most apparent when it comes to arms control.

For example, under the second strategic arms limitation treaty (SALT II) agreed to in 1979 by Jimmy Carter and Leonid Brezhnev but never approved by the U.S. Senate, a limit would have been placed on the number of American and Soviet missiles that carry MIRV-type multiple warheads of the variety that most threaten land-based missiles. It also would have, in effect, limited the number of warheads to the number already tested aboard each missile.

Thus, former Carter administration officials and other arms control advocates argue that the best way to reduce the threat to a future MX and to the two countries' retaliatory forces is to get back to arms control. They argue that with such provisions, the Soviets would not be able to overwhelm the kind of shell game the Air Force wants, which in its original plan would have involved shuttling 200 MX missiles between 4,600 shelters in Utah and Nevada.

On the other hand, there has never been any public indication from Moscow that it would accept MX, without a new Soviet buildup in reaction, even if the United States suddenly decided it would ratify SALT II. Furthermore, the Reagan administration has made clear its view that the United States must first begin to beef up its nuclear forces, match any real or perceived disparities with Moscow and then, maybe, think about going back to strategic arms control.

Administration officials also feel that if there is to be any eventual success with Moscow on what they call real nuclear arms reductions, then the United States must also go ahead with new weapons so they have a strong hand and possible bargaining chips at the negotiating table.

The MX as it is now being discussed, however, is not the big earlier Air Force plan of 4,600 shelters. That has been ruled out by a Reagan administration that didn't like that plan's Rube Goldberg look and Carter-era ancestry. Nor does the Reagan White House want to tussle any more than is necessary with opposition from powerful friends and political forces in the west, including the Utah-based Mormon Church. What reportedly is being discussed now is a scaled-down version involving 100 missiles and 1,000 shelters based in a part of Nevada where there is less opposition.

The problem here, however, is that such a scaled-down system is more likely to be overwhelmed by a much less extensive Soviet buildup than that required for the full system.

There are two potential answers. One is to keep expanding the system, which will make it eventually look like the original Air Force idea. The other, which has powerful supporters, is to keep the missile-shelter deployment limited and eventually put an ABM defense around it, something which advocates say would at least complicate Soviet targeting further and be the equivalent of perhaps two or three times as many shelters.

The problem with ABM, however, is that it would be very expensive and nobody is sure it will work. In 1969, Congress, after a long and bitter debate, came within one vote of rejecting a very limited ABM deployment. If that debate were reopened, it could divide public opinion once again over a military issue and cost this administration the public consensus and confidence that it now seems to have on defense issues.

For Moscow, MX in any form presents a real problem because the new U.S. missile is being advertised as a real silo-buster, accurate enough to fling 10 individual atomic warheads at 10 different silos and hit them. If this is true, however, it could put a potential hair trigger on nuclear war because both sides may well put a premium on firing first rather than losing their land-based missile force.

Supporters of MX argue that Moscow is apt to believe the MX capabilities and thus is less likely to keep on investing in new land-based missiles at fixed locations to overwhelm MX when those missiles would themselves be vulnerable to a first strike by MX.

Some specialists argue that Moscow's response to MX therefore is likely to be a similar system of mobile missiles moved around to shelters or around the countryside and that ultimately both superpowers armed this way would be more secure and thus more apt to reach some limitations.

There are already some intelligence reports that Moscow may be experimenting with MX-style missile bases. Yet, to get to this point, both sides will have to invest huge sums of money and could wind up at the same rough balance of power that exists today if invulnerability proves elusive or if today's missiles aren't really vulnerable to begin with.

Aside from a scaled-down MX deployment, another option that may still be under White House consideration involves putting off an MX decision for a few years while the search goes on for a new, smaller common missile that could be used by both the Navy and Air Force in a variety of safer mobile land, airborne or undersea launching schemes. If there is, in fact, no good way to base MX now, then perhaps it makes sense to take some more time, while deploying new bombers, submarine missiles and air-launched cruise missiles about which there is less controversy.

But here, too, some specialists believe there could be a considerable political liability for President Reagan in the sense that he would run the risk of being tagged by conservatives as the president who killed MX just as Carter was probably hurt politically by his decision in 1977 to kill the B1 bomber.

Reagan's statements last week about MX that "I don't know where we're going to put it, but we're going to have it" suggest that he is committed to the missile itself. However those remarks, coming after months of intense study about where to base this missile and just two weeks or so before he is expected to announce his decision, also reflect the basic problem: that there is no obviously good solution.

Finally, there is the question of whether the existing U.S. missiles are really vulnerable. This is not a new question, though it is getting some new attention as the crucial MX decision nears. An editorial in a conservative defense journal recently raised this issue and there is known to have been some preliminary discussions at high level with the Strategic Air Command about new tests of missiles over a path more close to the north-south route that would be flown in wartime to check out doubts about missile accuracy generally.

But there is virtually a zero chance that any officials high in the Reagan administration, or even former Carter administration officials, are going to stand up publicly and claim the U.S. missiles are not vulnerable. The Carter administration got military backing for the SALT II treaty in part by pledging to go ahead with the MX project.

The official view is that Moscow's missiles have the proper combination of accuracy, explosive power and quantity to make a successful attack at least theoretically possible. The argument against vulnerability is mostly an operational one: that neither Moscow nor Washington has ever tested missiles over the polar routes they would fly in wartime; that wind, weather, gravity and other factors would throw missiles off course; that a huge strike required to wipe out Minuteman would not only be suicidal but extremely difficult to coordinate.

There is, however, no way to prove this and the government holds most of the cards in its ability to say that secret data on Soviet missile tests proves their case. The public will never see that data and even if it did it would be hard to make sense out of knowing where Soviet missiles landed in tests unless the public knew what the real target was. Even then, the wartime conditions and flight paths for hundreds of missiles would be totally different for both sides than it is in isolated tests of single missiles.