As American and Soviet negotiators jockey for position over possible new arms control agreements, a key existing agreement faces an uncertain future.
This is the Anti-Ballistic Missile treaty (part of the 1972 SALT I accords), which comes up for review this fall. There is growing pressure in this country to change, if not scrap, the ABM treaty, a move some fear could undercut the current strategic arms reduction talks.
Critics of the treaty say the USSR already is violating it in spirit if not letter, and warn the United States has unwisely fallen behind in developing ballistic missile defense systems that could reduce the vulnerability of its land-based intercontinental missiles.
The Pentagon is planning to spend a lot more money to develop such systems, and an interagency review committee within the Reagan administration is looking at the ABM treaty for possible changes. Such changes, if in fact put forth by the administration, could have serious domestic and international political consequences, observers feel.
The dense-pack basing scheme for the MX missile eventually will need some form of ballistic missile defense, its advocates acknowledge, and - depending on the arms control mood in Congress - this could affect the future of the MX on Capitol Hill.
It is pointed out any US attempt to weaken the ABM treaty would hand the Soviet Union plenty of public relations ammunition as it continues to court the ''no-first-use'' and nuclear ''freeze'' movements here and abroad.
Existing ballistic missile defense systems include radars, launchers, and missiles designed to rise up and destroy attacking ballistic missiles. The Soviet Union has such a system in place around Moscow. The US began deploying a similar system (called ''Safeguard'') around the Minuteman missiles near Grand Forks, N.D., but the safeguard system is no longer active.
Under the ABM treaty, each country is limited to one such system. The immediate problem for the US is that Grand Forks is not a suitable site for the MX. If, as many experts expect, the MX needs a ballistic missile defense system to survive a first strike, the ABM treaty would have to be changed to allow the US to have its missile defense system somewhere other than Grand Forks.
''It might mean some change in the (treaty) protocol, which details things specifically,'' says a spokesman for the US Arms Control and Disarmament Agency (ACDA). ''But that shouldn't be an extraordinarily difficult thing to do for heaven's sake . . . or would it? Who knows?''
Officials from the ACDA and the State, Energy, and Defense Departments are pondering this question in preparation for the ABM treaty review, likely to take place in October. ''It isn't clear at this point as to what's going to happen,'' says the ACDA official.
What is more clear is the growing feeling in the Pentagon and among some defense advocates that the USSR may be pushing beyond the bounds of the ABM treaty and that the US must catch up.
In its report to Congress this year, the Joint Chiefs of Staff warned of a new Soviet phased-array radar being built around Moscow to strengthen or possibly replace existing missile defense systems.
In a Heritage Foundation report this week, former Defense Intelligence Agency analyst Guy Hicks notes an ''enormous loophole'' in the ABM treaty allowing the USSR to build more battle-management radars. ''Since the adoption of the ABM treaty, the Soviets have not only exploited this loophole to their advantage, but have actively upgraded their ABM capability in direct violation of the specific terms of the treaty,'' he writes.
Senate Armed Services Committee chairman John Tower (R) of Texas said recently the MX missile would no doubt require a defense system, suggesting he was not adverse to reexamining the ABM treaty if necessary.
Earlier this year, Sen. Pete V. Domenici (R) of New Mexico, in an article in Strategic Review, suggested that ''continued adherence to the ABM treaty simply represents an outdated legacy, and that this adherence may work to the detriment of rational US strategic force planning.''
Senator Domenici detailed the advances in technology that could lead to an ''overlay'' defense system able to detect and destroy enemy missiles outside Earth's atmosphere as well as at lower altitudes. Former Defense Intelligence Agency director Daniel Graham says a system of low-altitude satellites with interceptor missiles could be operating before the end of this decade.
The Pentagon has named a committee of scientists to study advanced missile defense technology and its treaty implications. In its defense budget plans, the Reagan administration wants to more than triple spending for ballistic missile defense between 1981 to 1984.
This may depend in large part on how Congress takes to the dense-pack plan for the MX, as well as possible ABM treaty implications.
For just as some see new missile defenses as a way to prevent nuclear war, many agree with former ACDA director and chief SALT I negotiator Gerard Smith who says, ''Deploying ABMs in large numbers would be a major destabilizing development.''