Reagan's 2 quandaries: missiles and the GOP

President Reagan this week announces his long-awaited decision on how to deploy the controversial MX intercontinental ballistic missile, thus setting off a debate with international ramifications which will influence arms control efforts as well as US military policy for years to come.

This is a watershed point in US strategic policy. On one side are those who want to continue the nuclear triad (land, sea, and airborne strategic weapons) as a means of deterring nuclear holocaust. Opposing this view are those who say land-based missiles henceforth can only be vulnerable and destabilizing, thus adding to the likelihood of nuclear war.

In announcing the new basing plan for the heavy and accurate missile, the President has three audiences in mind: (1) the US Congress, which is extremely skeptical and insists on a survivable home for the MX before spending billions of dollars, (2) the Soviet Union, whom the administration believes will be more forthcoming in arms control negotiations if the United States shows determination in rearming, and (3) the NATO allies in Europe, who would be loath to accept US-made Pershing II intermediate-range missiles on their soil if the United States hesitated to deploy the MX here in this country.

In the years since the MX (''missile experimental'') was conceived, there have been more than 30 plans for how to base it. The missile itself is a big advance over the 1,000 Minuteman III missiles currently deployed. The MX weighs 192,000 pounds, is 71 feet long, and carries 10 warheads (compared with just three for the Minuteman).

The key issue in basing the MX is the US ability to survive a Soviet first strike and be able to retaliate with such force that such a strike would never be launched in the first place. Among the basing proposals that have been offered over the years are small submarines, aircraft that could patrol for days , and the ''race track'' pattern favored by the Carter administration.

The Pentagon now favors the ''closely spaced basing'' system commonly called ''dense pack.'' Here, the missiles would be placed in hardened silos in close proximity so that most attacking warheads would suffer from the ''fratricide'' effect. This means enemy warheads, forced to strike with unattainable precision and timing, would destroy each other through blast, debris, and radiation, leaving most MX missiles unharmed.

Critics argue that the Soviet Union could ''pin down'' the closely spaced missiles by exploding a series of relatively small nuclear weapons above the MX groupings; it could use its hefty ICBMs to drop very large nuclear warheads on the missiles, disabling several at a time, even in their ''superhardened'' silos; or it could spread nuclear land mines over the MX field.

Air Force officials say critics overestimate Soviet capabilities to counter the MX. This raises the question of uncertainty and whether such uncertainty makes a nuclear war more or less likely. Proponents of the MX say it would increase superpower stability since the Soviet Union would be less confident that it could destroy US land-based missiles.

Opponents say the MX - being accurate enough to hit Soviet missile silos while remaining relatively vulnerable - would encourage Soviet leaders in a time of crisis to launch their missiles before losing them to a US first strike.

The US also is developing the Trident II (D-5) missile, to be deployed on nuclear submarines. This will be as heavy and accurate as the MX, but less vulnerable to a first strike.

Some MX opponents say the US should move away from land-based missiles and accelerate the Trident program instead. The administration contends that all three legs of the nuclear triad must be maintained, especially since control of submarines is more difficult.

How - indeed whether - to deploy the MX missile thus forms a key juncture in the arms race. The basing decision is certain to raise questions about possible violations of SALTs I and II, particularly if an antimissile defense system is eventually included (as many experts, including key supporters, suspect will be necessary).

The MX is likely to be a key target of Congress for budgetary as well as military reasons. Lawmakers earlier this year approved a portion of the administration's request for MX funding only by insisting that a permanent basing mode be presented before Dec. 1. The House defense appropriations subcommittee last week approved by a single vote money to buy the first five MX missiles, but full House approval remains uncertain. In the Senate, Ernest F. Hollings (D) of South Carolina says he has tallied enough votes to block production of the MX, total price of which could reach beyond $25 billion for a total of 100 missiles.