Secretary of State George P. Shultz gave strong backing yesterday for U.S. and Soviet guarantees of continued adherence to the 1972 Antiballistic Missile (ABM) Treaty, saying it would enhance stability for both superpowers.

A lengthy period of guaranteed nonwithdrawal from the ABM Treaty is expected to be among the central issues concerning President Reagan and Soviet leader Mikhail Gorbachev in next week's summit.

Speaking on CBS-TV's "Face the Nation," Shultz repeated that Reagan will not abandon his Strategic Defense Initiative (SDI) in the meetings with Gorbachev.

Shultz talked of the need for a period of "predictability" that would come with a substantial period of "nonwithdrawal" from the ABM pact, which sought to prevent a race in defensive missiles. "One method of providing the predictability and stability that both sides want . . . is to have a period in which we agree not to withdraw from the ABM Treaty or to exercise our right of a six-month notice or to deploy," Shultz said.

The Soviet Union is seeking a 10-year nonwithdrawal agreement plus some controls on space-based SDI tests as part of its attempt to seek limits on U.S. development of the SDI antimissile defense system. Reagan agreed at last year's Reykjavik summit to a 10-year nonwithdrawal period under tight conditions, including the elimination of all U.S. and Soviet ballistic missiles. The current U.S. offer is a seven-year nonwithdrawal period.

Limits on strategic, or long-range, nuclear weapons as well as defensive weapons will be the two key issues at the summit. Reagan and Gorbachev are also expected to sign an intermediate nuclear forces (INF) treaty to eliminate all U.S. and Soviet medium- and shorter-range nuclear missiles from Europe.

Shultz's strong endorsement of a nonwithdrawal period appeared to be signaling a willingness to negotiate on this issue. He said the need for a period of stability was "just as important for us as it is for them because probably, right at the moment, their ability to field what we think of as an inferior form of strategic defense is greater than ours. We don't want to reduce our offensive system unless we have some notion of stability, just as they don't."

Officials in the State and Defense departments have concluded that the limits on SDI being sought by the Soviets would interfere with only a handful of technological tests between now and 1995. They said it might be possible to negotiate limits on SDI testing without fatally compromising the program.

Reagan returned to Washington yesterday from a Thanksgiving weekend at his California ranch. A spokesman said the president plans to spend much of this week lobbying on behalf of the INF treaty.

Senate conservatives have threatened to try to block ratification. Sen. Steve Symms (R-Idaho) said yesterday that he had "very serious concerns about it." Symms, also appearing on "Face the Nation," said he was concerned about the verification language in the INF treaty and would probably seek to have the treaty amended in the Senate.

Shultz defended the verification procedures under the treaty yesterday, describing them as a "successive set of layers" that left cheating possible, but not probable.