The United States and the Soviet Union resumed negotiations on nuclear and space weapons today in a critical six-week round of talks that will set the stage for the Nov. 19 summit meeting here between President Reagan and Soviet leader Mikhail Gorbachev.

The Soviet delegation chief, Viktor Karpov, led a 23-man team into a two-hour session at the U.S. arms control agency mission. Upon arrival, he told journalists that he had instructions to conclude an accord "that will lead to the nonmilitarization of space and the termination of the arms race on Earth."

Karpov said chances for progress "depend on our negotiating partners. If they show willingess to reach effective solutions, then there will be progress; if they don't, there won't."

The Reagan administration has charged that the Soviets have engaged only in propaganda thus far and have failed to put any new proposals formally on the negotiating table since the talks began in March.

As the delegates posed for photographs, Max Kampelman, head of the U.S. delegation, remarked -- apparently with a mixture of humor and hope -- that "we are all aware that the Soviet Union is coming in today with important new positions."

Karpov smiled but did not reply. Later, he shook hands with Kampelman and said, "Let's hope that helps."

The Americans are still waiting for the Soviet Union to make the first move to break the stalemate in the three-part negotiations on strategic, medium-range and space-based defensive weapons. The talks have achieved no noticeable progress since opening March 12.

U.S. delegation sources said they believe that Gorbachev is now likely to make a greater impact on the negotiations than had been detected in the first two rounds. The replacement as foreign minister of Andrei Gromyko by Gorbachev's protege Eduard Shevardnadze is expected to produce significant changes in substance as well as style. Gromyko was known for his unyielding views on arms control.

Shevardnadze is scheduled to meet Reagan in Washington on Sept. 27 and any new shifts in the Soviet bargaining positions may become more evident at that time.

U.S. negotiators are intent on using the current round, which will end Nov. 1, to explore Gorbachev's promise of "radical proposals" to reduce nuclear forces in exchange for restrictions on Reagan's Strategic Defense Initiative, the space-based antimissile system also known as Star Wars, according to senior U.S. officials.

Despite Reagan's stated refusal to bargain away SDI in exchange for deep cuts in nuclear missile forces, U.S. negotiators have been authorized to seek a clearer definition from the Soviets on what constitutes research into space weapons in an effort to get the talks moving.

In an interview with Time magazine and talks with U.S. senators in Moscow this month, Gorbachev said the Soviet Union was prepared to make radical cuts in strategic nuclear arms if the United States agreed to halt SDI development.

The Americans would like to see the Soviets, in this round, make numerical proposals to match their negotiating positions with pronouncements of their leaders.

In April, Gorbachev told Warsaw Pact leaders that Moscow was prepared to trim long-range missile forces by 25 percent. Since then, he and senior Soviet military officers have hinted Moscow would go much further, perhaps to reductions of 40 percent or more, if the United States accepted limits on space weapons.

Soviet military officials also have said that Moscow is willing to accept similar limits on nuclear warheads, which the United States has sought. But so far, Soviet negotiators have not offered new numbers or clarified their position on warhead limits.

The U.S. position calls for a one-third cut in strategic arsenals to 5,000 warheads for each side, combined with a limit on total explosive power on land-based missiles, to thwart one side from being able to destroy the other's missiles force.

In his sessions with Time and the U.S. senators, Gorbachev also said that while "fundamental science" research could not be stopped, the design, testing and deployment of weapons should be banned.

As a result, U.S. officials said, the ban on all research urged by Soviet negotiators throughout the first two rounds now appears untenable.

Another area of potential movement this round is reorienting the nuclear forces toward less threatening postures, a long-term Reagan administration goal.

Near the close of the second round in July, Soviet delegates floated the idea of negotiating percentage limits on the bombers, submarines and land-based missiles that deliver nuclear warheads. The Americans intend to use these talks to delve further into Soviet thinking on this because it could indicate a new willingness to shift the emphasis from powerful missiles toward a more stable mix of launchers.

The Soviet Union's heavy land-based missiles, which most threaten the United States, constitute 60 percent of its nuclear force, according to U.S. delegation sources.

If the Soviets accept a ceiling on long-range missiles as low as 40 percent and make equivalent cuts in warheads, there could be dramatic reductions in the most dangerous kinds of first-strike nuclear weapons, U.S. officials said.

The Soviets have a decided edge in land-based missiles. But the United States has superiority in bombers and submarine-based missiles.