The small, mobile Midgetman missile being developed to strengthen the U.S. land-based nuclear deterrent is too small to carry some planned payloads to military targets throughout the Soviet Union, according to a General Accounting Office report released yesterday.

For Midgetman to lift both a 1,000-pound nuclear warhead and "penetration devices" to help the warhead through Soviet defenses, the GAO said, the Air Force will have to modify the Midgetman's design or base the missile in northwestern states, abandoning plans to locate them in the Southwest.

In its first critique of the missile, the GAO questioned Midgetman's range along with a number of "significant" technical and operational problems that it said could delay the weapon's scheduled deployment in the early 1990s.

The report also set the first official price tag for the system, estimating costs of $44 billion for a force of 500 missiles that would be towed by armored vehicles designed to withstand nuclear blasts. However, the GAO said many factors affecting Midgetman's final cost have yet to be determined. For example, the Pentagon has yet to set the number of Midgetmen to be deployed, so the Air Force has had to plan on a force of "from 250 to over 1,000," the GAO said.

The findings of the congressional watchdog agency are expected to be cited by those members of Congress demanding a fresh appraisal of Midgetman, which is touted by proponents as the land-based intercontinental ballistic missile of the future. The House voted $150 million more than the administration requested, and the Senate previously approved, to finance development of Midgetman in fiscal 1986, and the battle over the budget is now being fought in a House-Senate conference.

Citing the practical problems of towing a 46-foot-long, single-warhead rocket around military bases, the GAO estimated that 20,000 workers would be needed to operate, maintain and guard a randomly dispersed force of 500 missiles carried by armored vehicles. Access to 4,000 square miles of "suitable" land would be required for daily operations and 8,000 square miles in periods of increased alert, it said.

Technical problems cited by the report include building a missile light enough to be easily moved, developing an affordable guidance and control system that can remain accurate while moving and designing an armored vehicle mobile enough to quickly transport missiles and strong enough to survive a nearby nuclear blast.

The chief operational problem noted by the report is the size of the missile, 15 tons, which does not provide enough volume to carry the needed propellant, guidance system, penetration aids and warhead, according to the GAO and Senate critics.

Current plans call for a missile to be deployed in the Southwest with a range of 6,000 miles and the capability of carrying a nuclear warhead and devices, such as decoys, designed to foil an antiballistic missile system.

The report said the missile's 1,000-pound payload capacity is "not sufficient" to carry both a reentry vehicle and penetration aids "without a degradation in range. To maintain range while carrying both packages will require modifications to the missile or basing in northern locations," the agency said.

Six locations in the Southwest are still being considered by Air Force as "optimum candidates" for vehicle-towed missiles because of their "large amounts of suitable land," according to the report. It said securing land for deployment could be "challenging and time-consuming" because of required environmental impact studies.

The report was well received by Midgetman doubters in Congress, including Sen. Pete Wilson (R-Calif.), who has argued for a larger missile capable of carrying multiple warheads as well as penetration aids.

"This version of Midgetman can't cut it," he said. "We have to go back to the drawing boards."

Rep. Les Aspin (D-Wis.), chairman of the House Armed Services Committee and a major backer of Midgetman, said none of the issues raised by the report are insurmountable, adding, "We're still in the process of developing it."