The Soviet Union may overcome a U.S. lead in nuclear weapons technology and expand its knowledge of controversial X-ray lasers if Soviet leader Mikhail Gorbachev ends his eight-month moratorium by renewing underground nuclear weapons testing, according to Rep. Les Aspin (D-Wis.), chairman of the House Armed Services Committee.

Aspin, in an interview, criticized the Reagan administration for summarily rejecting Moscow's call for a superpower moratorium on nuclear testing and for not adequately studying "the tradeoffs" that would come from a permanent halt to Soviet tests.

Since Gorbachev's announcement of a temporary Soviet moratorium last summer, President Reagan has made it clear he would not stop the U.S. testing program. The president has said that the United States had to conduct tests periodically to be certain the U.S. stockpile is an effective and reliable deterrent.

In the eight months since the Soviets stopped testing, the United States has conducted eight tests. The next shot at the Nevada Test Site is due any day, according to informed sources.

Gorbachev on Saturday called for a superpower summit to work out an end to testing; he also repeated an earlier pledge to continue the Soviet moratorium until the first U.S. test after March 31.

An administration official said yesterday that because of White House determination to avoid being pressured into joining the Soviet moratorium, some "countervailing arguments" on the benefits of such a step did not get a full hearing by U.S. policy-makers. But Gorbachev's latest statement was "a poorly thought out propaganda gesture" that the Soviet leader knew would not be accepted, the official added.

Assistant Secretary of Defense Richard N. Perle, asked recently if he feared the Soviets would catch up to the United States in warhead design, replied "I'm not unhappy if the Soviets produce more reliable weapons."

One expert on Soviet weapons who sometimes serves as a consultant to the U.S. government called the administration's decision to ignore Gorbachev's offer "a sign of a bankrupt arms policy." The administration, he said, "gave Gorbachev a propaganda victory and prevented a chance to complicate [the Soviets'] moving to higher-tech nuclear designs."

Aspin said that a resumption of underground nuclear testing might allow the Soviets to reduce the size of nuclear warhead components and possibly double the number of warheads carried on the Soviets' biggest long-range missile, the SS18, or its successor, which is now in development.

Currently, the SS18 carries "10-plus warheads," according to a Pentagon publication. In the future, with "warhead packaging efficiency close to that of the [U.S.] MX," Aspin said, an SS18-sized missile could carry 24 warheads, thus complicating U.S. defenses and arms control efforts.

Although U.S. officials have testified over the past year that little is known of the details of Soviet nuclear weapons, the general belief is that they use more nuclear material and have less sophisticated electronic components than those of the United States.

One result is that the Soviets must have much larger warheads on their missiles. If they can reduce the size of their nuclear packages, the Soviets could put many more warheads on their large missiles, Aspin said.

Steven Meyer, an expert on Soviet military systems at MIT, said that engineering shortcomings are a major problem in Soviet nuclear weapons building. If Gorbachev can modernize Soviet industrial capability, Meyer said, "we can expect smaller components with higher reliability" in nuclear warheads.

"They ultimately will need a series of [underground test] shots to make sure they work," he added.

At a Senate Armed Services subcommittee meeting March 25, Lt. Gen. James A. Abrahamson, director of Reagan's Strategic Defense Initiative, pointed out another area where renewed Soviet testing could provide major gains for Moscow.

Abrahamson said the Soviets may be ahead in research on X-ray lasers released by nuclear explosions, which could be used to destroy space-based components of a "Star Wars" system or U.S. nuclear missiles fired in retaliation against a Soviet attack.

Soviet publications first carried articles about X-ray lasers in 1974, Abrahamson said, and the Soviets conducted an X-ray laser technology test in 1982 "that we will not be able to do until 1987."