ISSUES '84; WHERE THE CANDIDATES STAND ON DEFENSE; MONDALE

Walter Mondale promises a military defense of the United States that will be ''smart, lean, and tough.'' This means a Pentagon budget that will continue to increase by 3 or 4 percent a year after adjusting for inflation, but one that would total $25 billion less than the Reagan administration is projecting through 1989.

Mr. Mondale charges that Reagan's defense effort has been too heavily weighted in strategic nuclear weapons. He would cut the MX missile - as vice-president, he was ''prepared to endorse'' it the racetrack-basing mode - and the B-1 bomber. He also opposes space-based missile defenses (except for more modest research), sea-based nuclear cruise missiles, and antisatellite weapons.

He would support air-launched and ground-based cruise missiles, the ''stealth'' bomber, new Trident submarine-launched nuclear missiles, and development of a new single-warhead, mobile intercontinental ballistic missile. All of these things, he says, would add to superpower stability because they do not form a first-strike capability.

In the area of conventional arms, Mondale would not allow the construction of new chemical weapons, which Congress has denied the Reagan administration, or the addition of two more aircraft-carrier battle groups to the US fleet.

Mondale criticizes the move toward ''horizontal escalation'' in time of conflict, particularly talk of attacking the Soviet fleet in port should conflict begin.

Mondale spokesmen admit, however, that it may be impossible to halt the already begun new carrier and B-1 programs because of the large sums already spent and widespread political support tied to construction jobs.

Mondale emphasizes the need to spend relatively more on military readiness and sustainability - ammunition, spare parts, and training - and less on new weapons. Still, he has not outlined where he would cut back on such things as the M-1 tank or advanced (and expensive) fighter jets.

Without describing exactly how, Mondale says he would ''make sure our allies in Europe and the Pacific pay their fair share for our collective conventional defenses.'' He promises to ''require that contractors stand behind their products with appropriate warranties,'' and says he will ''demand independent testing for weapons.'' The Reagan administration has resisted an independent testing office and broad, congressionally-mandated weapons warranties.

Without being specific about reforming procurement procedures and practices, the Democratic challenger says he ''will make sure that the arms we buy cost no more than they should.''

Mondale says Mr. Reagan ''has no coherent defense strategy.''

Mondale does not say how or whether he would change US commitments abroad. But he promises to reform the Joint Chiefs of Staff (JCS) ''to ensure better management of our defense buildup, a better match between strategy and resources , and better-prepared fighting forces.''

He would make the JCS chairman a full member of the National Security Council and also says he would involve senior military officers more in planning and carrying out military missions.