The Soviet Union has refused to soften its formal demand at the Geneva arms negotiations for an absolute ban on research into space-based missile defenses despite earlier suggestions by Soviet leader Mikhail Gorbachev that laboratory work might be allowed, according to U.S. senators monitoring the talks.

Moscow's reluctance to confer any legitimacy on President Reagan's Strategic Defense Initiative, or "Star Wars" research program, remains the major obstacle to a breakthrough at the Geneva negotiations, a bipartisan group of six legislators reported after intensive consultations here with U.S. and Soviet negotiators.

Even so, the senators concurred in an interview that the current negotiating round, which ends Friday, has produced the most promising and important exchanges held by the two delegations since the Geneva talks opened in March.

The senators' positive assessment was based on the latest Soviet proposal, which showed a new willingness by Moscow to undertake drastic cuts in strategic nuclear arsenals. They also noted that the coming meeting here between Gorbachev and Reagan on Nov. 19 and 20 had clearly focused the negotiators' attention on preparing for a possible summit accord that might stake out guidelines toward a future arms agreement.

The senators, members of the so-called Arms Control Observers Group following the negotiations, included Republicans Ted Stevens (Alaska), Malcom Wallop (Wyo.) and Pete Wilson (Calif.), and Democrats Sam Nunn (Ga.), Albert Gore Jr. (Tenn.) and Edward M. Kennedy (Mass.).

"It's an unusually delicate moment, replete with possibilities and opportunities," Gore said, referring to the summit's potential impact on the negotiations. He and Kennedy expressed the hope that it would emulate the 1974 Vladivostok summit, when then-president Gerald Ford and Soviet leader Leonid Brezhnev agreed on a general framework that culminated in the SALT II treaty five years later.

But Stevens and Wilson warned that the complexities inherent in the triple areas of intercontinental nuclear weapons, medium-range missiles and space and defensive arms were too formidable to expect that "a precooked deal" could be reached at the summit.

"Any breakthrough to be worked out at a two-day meeting is not only wishful thinking but suspicious," Wilson said.

Kennedy, however, insisted that the American public would not be satisfied with "minor deals on fishing rights, but would judge the summit on progress in arms control."

A central question impeding progress in the talks is whether the Soviets will allow research to proceed in space-based defenses while offensive nuclear arsenals are reduced in an overall arms agreement.

Nunn said Gorbachev told him and other senators visiting Moscow earlier this year that the Soviet Union was prepared to allow "fundamental science" or laboratory research to continue since such work could not be verified.

But so far, this view has not been put forward in Geneva, thus thwarting any chance for a potential compromise. Soviet negotiators continue to insist on banning scientific research and the testing and development of "space-strike arms."

The ambiguity in the Soviet negotiating position was underscored on Oct. 4, two days after the Soviets had presented their formal position at Geneva, when Gorbachev said at a press conference in Paris that he stood by a statement he made to Time magazine in September that fundamental research could proceed on defensive space systems.

Soviet negotiator Yuli Kvitsinsky used the same phrase after the press conference in response to a question from The Washington Post, specifying that fundamental research is carried out in a laboratory.

The senators, who met with the Soviet delegation Friday night, said the current six-week round of talks has been dominated by sharp give and take over intentions and details of the latest proposals by Gorbachev during his recent visit to Paris.

The Soviets have called for a 50 percent cut in strategic missiles and bombers, with a ceiling of 6,000 nuclear "charges" under which there would be a 60 percent limit on warheads fired from land-based missiles, aircraft bombers or submarines.

The standing American offer would limit strategic ballistic missiles to 5,000 warheads on each side and reduce U.S. and Soviet intermediate-range missiles to the lowest possible parity.

The senators welcomed the Soviet offer because it narrows the gap with the U.S. position and also would mean a limit of 3,600 warheads carried by Soviet intercontinental missiles, a substantial reduction in the long-range threat feared most by U.S. strategic planners.

But the senators also emphasized numerous points in the Soviet proposal that remain unacceptable to the United States and its allies.

In particular, Moscow has resurrected its 1969 definition, later discarded in SALT I and II negotiations, of "strategic" weapons as those capable of hitting U.S. or Soviet territory. This would have the effect of including U.S. missiles and bombers based in Europe in the "strategic" realm, while neglecting about 2,000 Soviet medium-range missiles and aircraft.

American negotiators reportedly had told their Soviet counterparts in vehement terms that the United States strongly opposes having to choose between systems to defend its allies and those that would defend the continental United States.

Moscow also wants to ban all "new" nuclear delivery systems that have not been flight-tested. The United States objects that such a freeze would proscribe its advanced but untested systems such as the Midgetman mobile, land-based ballistic missile, the highly accurate D5 missile carried aboard the Trident II submarines, and the Stealth bomber, while the Soviets would be permitted to proceed with their latest SS24 and SS25 intercontinental-range missiles and the Blackjack bomber.

The Soviets also have demanded a ban on long-range cruise missiles, which, if defined as in the past, would mean removal of systems deployed during the past two years in Britain, Italy and Belgium and the blocking of other cruise missiles to be deployed in the Netherlands and West Germany by 1988.