The team of U.S. negotiators had just put aside papers, pencils and briefing books after conducting their final brainstorming session on the eve of the first detailed nuclear arms talks between the superpowers in 15 months.

Max Kampelman, head of the U.S. delegation to the Geneva arms control talks, turned to the U.S. senators who had been watching in meek silence as the Americans rehearsed their strategy before the initial meeting with the Soviets.

"Don't forget," Kampelman admonished in a friendly way, "I need to have your views, to hear you guys speak out from time to time. Don't be afraid to give your own opinions."

Kampelman's remarks acknowledged the importance of perhaps the most extraordinary participants in the embryonic dialogue with the Soviets on space, strategic and intermediate nuclear arms: the Senate Arms Control Observers Group.

Unlike any previous negotiations with the Soviet Union, the U.S. Senate will be intimately involved in the bargaining process from the start instead of merely voting on a package deal at the end.

Bringing the Senate into the negotiating process could ensure solid bipartisan support for an eventual agreement, observers say, or it could unleash more conflicts that splinter U.S. positions and possibly jeopardize the talks.

"Our long-term objective," the senators said in a joint statement here, "is to avoid a recurrence of the problems of the 1970s, when three successive arms control treaties, signed by three presidents, were never approved for ratification by the Senate."

The 10-member observer group was established by a Senate resolution, passed unanimously Jan. 3, to keep the body informed of progress in the negotiations and to offer advice to the administration.

Cochairmen are Republicans Richard G. Lugar (Ind.) and Ted Stevens (Alaska) and Democrats Sam Nunn (Ga.) and Claiborne Pell (R.I.). Other members include Malcolm Wallop (R-Wyo.), Daniel Patrick Moynihan (D-N.Y.), Edward M. Kennedy (D-Mass.), Albert Gore Jr. (D-Tenn.), John W. Warner (R-Va.) and Don Nickles (R-Okla.).

The Soviet Union, annoyed by the string of unratified treaties, has apparently welcomed a more prominent role by Congress. Soviet negotiators have expressed a desire to meet often with the senators here.

For the Reagan administration, the benefits of linking five Republican and five Democratic senators so closely with the destiny of the negotiations appear to outweigh potential risks.

Politically, the administration expects to be spared a lot of the agony incurred by its predecessors if an arms deal comes up for ratification. Senators from both parties presumably would be more cautious about challenging an agreement shaped along the way by respected peers in Geneva.

Moreover, the administration also believes it will gain more bipartisan support during the talks for funding votes on controversial projects such as the MX missile and the Strategic Defense Initiative.

The senators, too, want to avoid nasty floor fights over arms control policy and to present a more united U.S. front than during earlier negotiations on strategic and intermediate nuclear weapons.

The Soviet Union broke off those talks in December 1983, when NATO began deploying Pershing II and cruise missiles in Western Europe to counter the Soviet buildup of triple-warhead SS20 rockets.

Despite the display of U.S. harmony, there are apprehensions about potential problems arising from the unprecedented involvement of Congress in what are viewed as the most complex and delicate negotiations ever undertaken with the Soviet Union.

The senators plan to keep one or two members in Geneva to follow the talks as closely as possible. In addition, they want to be kept informed of private contacts, which provide a chance to drop overtures beyond the formal realm of the bargaining table.

Such unofficial exchanges led to the "walk in the woods" deal on medium-range missiles, later spurned in both capitals, that was reached privately by Paul Nitze and Yuli Kvitsinsky in the earlier negotiations.

In Geneva, the senators will receive top-secret briefings from Kampelman and join staff meetings and some encounters with Soviet negotiators. They have office space and a budget of $500,000. In Washington, the senators will have access to all cable traffic to help them stay abreast of the talks.

One fear among U.S. delegates is that sensitive position papers or accounts of the negotiations might be leaked because of the extensive sharing of information with the Senate. The United States and the Soviet Union have agreed to a strict confidentiality rule for the talks.

"If leaks start to occur and they are traced to the Hill, there will be quick remedial action," a member of the U.S. delegation said.

Another worry is that if the talks proceed slowly, as many people expect, the senators will choose sides publicly in any internal debate about new initiatives, thus aggravating the spectacle so apparent in Reagan's first term of a government badly divided on arms control policy.

The senators have praised the U.S. negotiating team and gone out of their way to insist that they will not do anything to disrupt the work of the negotiators.

Kampelman, who will handle the talks on space and defensive arms, is a Democratic lawyer who worked for the late Hubert H. Humphrey and has excellent contacts on Capitol Hill.

John G. Tower, the chief negotiator on strategic or long-range nuclear weapons, served 24 years in the Senate and headed the Armed Services Committee. His former colleagues say he should enjoy strong support.

Maynard Glitman, a career Foreign Service officer who is negotiators for intermediate-range arms, has little experience with Congress. But several senators cited approvingly his professionalism and the strong backing he got from Nitze, his former boss.

While insisting that they will not interfere with the negotiators, several senators said they would not hesitate to go to the president with "new ideas" if a deadlock develops.

But Democrats Gore and Nunn, who were instrumental in compelling the administration to present a more flexible "build-down" proposal at the previous strategic arms reduction talks (Start), said they did not foresee circumstances where the Senate would be urging new positions on the administration.

"It was a different situation then," Nunn explained. "The early Start proposal was unrealistic and the MX vote was close, so the administration had to bend. Those two facts are not likely to converge in the future."

If anything, the Senate's close involvement in the current talks has aided the administration in winning over supporters for the controversial missile. Both Nunn and Gore said they now planned to vote for a limited number of MX missiles.

Similarly, several senators said Congress was becoming more favorably inclined toward the $26 billion space research program during the next five years because of the strong feeling that the prospect of the space-based Strategic Defense Initiative antimissile system, also known as Star Wars, brought Moscow back to the negotiating table.

But at some point in the talks, the senators said, the administration would have to consider dispensing with a space-based defense program if that would entice the Soviet Union to accept radical reductions in offensive nuclear weapons.

Another possible area of conflict could be the issue of a moratorium on nuclear weapons deployment, an initiative that the new Soviet leader, Mikhail Gorbachev, hinted at in his first major speech.

Kennedy pointed out that 40 senators voted for a nuclear freeze. The issue could be revived and become a factor in the negotiations if the Soviets try to split the United States from its European allies, and Congress from the administration.

But U.S. negotiators have ruled out a freeze on missiles while the Soviets hold superiority in numbers.