Washington — The United States moves forward in the nuclear arms race this week as the Air Force formally presents its proposal for how to base the new MX missile.
Beyond the details of this so-called ''dense pack'' scheme, the most important issue is: Will the MX increase or decrease the stability of the nuclear balance? Will it make nuclear war by design or accident more or less likely?
Even among its strongest supporters, there is doubt that cramming MX missiles together can meet the key test of first-strike survivability for long without additional controversial features. Included here might be deep underground basing, a deception plan (reminiscent of the rejected ''race track'' basing notion), and a ballistic missile defense system.
Critics say these could violate the spirit if not the letter of the Anti-Ballistic Missile Treaty as well as SALTs I and II.
With dense pack (what the Air Force calls ''closely spaced basing''), groups of 10 missiles would be placed about 2,000 feet apart in an area of 10 to 15 square miles. Each missile (100 in total) would be placed in a ''superhardened'' silo able to withstand up to 10,000 pounds of pressure per square inch (five times the strength of existing silos).
In order to successfully attack these missiles, Pentagon experts say, the Soviet Union would have to land warheads directly on each missile with precise timing. More likely, US planners believe, such a crowded attack would result in ''fratricide'' in which incoming missiles would be destroyed or thrown off course by their own blast, radiation, or cloud of debris. This would leave most of the MX missiles unharmed and able to rise in counterattack, dense pack advocates say.
''In theory, you can design responses which pose a fatal threat to CSB (closely spaced basing),'' says Colin Gray, president of the National Institute for Public Policy and a strategic arms expert who has been advising the Pentagon on the MX. ''But the kind of performance required of the Soviets to do that is such that it really stresses them where they are weakest.''
Dr. Gray (who favors building the MX) acknowledges that there are ''residual uncertainties'' and ''potential vulnerabilities'' with dense pack basing. ''No one can really model in precise detail what the attack environment would look like,'' he says. ''We don't have any experimental data for overlapping fireballs.''
Nor is either side likely to have detailed information about the effects of fratricide as long as they refrain from above-ground testing of nuclear weapons.
But it is this uncertainty itself, it is argued, that would prevent the Soviet Union from launching an attack against US intercontinental ballistic missiles and thereby increase stability between the two nuclear superpowers.
On the other side are those who say the dense pack basing of MX leaves it vulnerable to attack. Also, critics argue that since the missile itself is extremely heavy and accurate, deploying it in any fashion destabilizes the nuclear balance.
''Given the accuracy that's now attainable with intercontinental-range missiles and the yields of the warheads, it's just not possible to design something which can't be successfully attacked if you can locate it,'' said retired Adm. Noel Gayler, former National Security Agency chief.
Admiral Gayler similarly criticizes the Soviet Union for building and deploying its SS-18 missiles, but he says that for the US to go ahead with the MX would simply lead to further arms escalation.
''One of the things that's so very difficult to get across, which I learned when I was responsible for target planning, is that when both sides have so many thousands of weapons, the relative force sizes don't make a lot of difference,'' says Admiral Gayler.
Critics of dense pack say the Soviet Union could ''pin down'' MX missiles that have survived a first strike by detonating warheads overhead, creating radiation and electromagnetic pulses. This would prevent the MX from being launched while the effects of fratricide subsided and allow the Soviet Union to launch a second strike at MX areas.
For this reason, Senate Armed Services Committee chairman John Tower (R) of Texas and other MX advocates say a ballistic missile defense system must accompany dense pack basing. Since this probably would violate the ABM treaty of 1972, Congress is expected to probe deeply as it weighs funding for MX production and deployment.
For the coming fiscal year, Congress has authorized money for the first five MX missiles, four less than the Reagan administration wanted. The first missiles are scheduled to be deployed in 1986, dependent on congressional approval of a basing plan.