A group of U.S. senators attending the Geneva negotiations on nuclear and space arms today welcomed the announcement of a U.S.-Soviet summit here as an opportunity to inject momentum into the talks, which have achieved little progress since they opened in March.

The senators also expressed hope that the rapid consolidation of power by Soviet leader Mikhail Gorbachev, as evident in the latest shake-up in the Soviet hierarchy, would enhance prospects for more reasonable initiatives from Moscow that could lead to an early arms control agreement.

Sen. Sam Nunn (D-Ga.) reported that while there have been "meaningful discussions" in the forums on intercontinental nuclear weapons and space defense arms, the Soviets have declined "to put any numbers on the table" that would demonstrate their desire to reach a compromise.

He said that in the third area of negotiations, covering intermediate- range nuclear missiles, the Soviet position has "actually moved backward" from the final offers in earlier arms talks that broke down when Pershing II and cruise missiles were deployed in Western Europe to counter Moscow's SS20 buildup.

One positive development is understood to be the less strident tone adopted by Soviet negotiators in this round. During the first six-week session that ended in April, Soviet delegation chief Viktor Karpov and his lieutenants spent much time indulging in harsh polemics that castigated U.S. positions.

Nunn and Sen. Ted Stevens (R-Alaska) are cochairmen of the Senate Arms Control Observers Group, which monitors the arms talks. They were joined at a news conference today by three Democrats active in arms control: Edward Kennedy of Massachussetts, Albert Gore of Tennessee and Gary Hart of Colorado.

Despite the absence of progress in the arms negotiations, Gore said it was premature to conclude that there is a total stalemate. "Neither side is simply stonewalling," Gore said. "A meaningful dialogue is under way, which could, over time, substantially narrow differences" between the two sides.

Citing a cause for cautious optimism, Gore praised two "common understandings" announced June 14 by U.S. and Soviet arms experts who meet regularly to promote the aims of the 1972 Antiballistic Missile Treaty and the 1971 accord to reduce risks of nuclear war.

Although details remain classified, the two sides agreed "to further enhance the viability" of the ABM treaty and to work more closely to prevent inadvertent war by keeping nuclear weapons from falling into the hands of would-be "nuclear terrorists."

The senators said they were convinced that the three U.S. negotiators now possess sufficient flexibility in their instructions to quicken the pace of the talks if the Soviet Union made a "good faith effort" to bridge the negotiating gaps.

Nunn urged the Soviets to come forward with "numbers on the table that would explain what they mean by significant cuts" in offensive nuclear weapons. He also contended that Moscow should stop criticizing U.S. plans for research into space-based defense that Soviet scientists "have been doing for a long time."

The senators, who met for five hours at an informal gathering last night with the Soviet delegation, said they stressed there was full bipartisan support for U.S. complaints about alleged Soviet violations of the ABM treaty, including construction of an early warning radar system stationed in Siberia, near Krasnoyarsk, and the illicit coding of missile test data.

The senators conveyed a strong impression that all eyes are now, as Kennedy put it, "focused on the coming summit" as a watershed for the fate of the arms negotiations. The current session of talks ends July 16, and a third round will begin in September and conclude shortly before the Nov. 19-20 summit, but no breakthrough is likely before Gorbachev meets President Reagan. During the private social talks with the senators, the Soviet negotiators gave no indication of imminent shifts in their hard-line public posture, nor did they betray any unease about the sudden appointment of a new foreign minister, Eduard Shevardnadze, who has little experience in foreign and security affairs.