A massive Soviet arms buildup puts "very much in question" the value of the 1972 Antiballistic Missile (ABM) Treaty, banning most defensive strategic weapons, an authoritative administration official said yesterday. He added that "it might be wise" to modify the ABM treaty in the years after President Reagan leaves office.

In a background briefing for a group of White House reporters, the senior official elaborated on the administration's latest public stance toward the Soviet Union, which Reagan outlined in his press conference Monday night.

The president emphasized his unwillingness to accept any restrictions on the development of his Strategic Defense Initiative (SDI) -- an attempt to invent a defense against incoming missiles which is the Soviets' chief target in the Geneva arms negotiations.

The official said Reagan will tell Soviet leader Mikhail Gorbachev when they meet Nov. 19-20 in the Geneva summit that the United States has fallen so far behind Moscow's offensive nuclear buildup that four decades of reliance on offensive deterrence "may not work in the years ahead."

Reagan will suggest the superpowers explore turning to defensive weapons, the official said yesterday.

The official briefing reporters yesterday offered this as the approach Reagan will take at the summit, but other officials have confirmed that the administration is still at the early stages of summit preparations.

In recent weeks senior officials have offered several explanations, some of them contradictory, of administration thinking and intentions.

Yesterday the senior official said Reagan will be making a series of public statements on superpower relations before the summit begins.

The official briefing reporters yesterday said Reagan intends to use the summit for a "thorough exchange" with Gorbachev on the doctrine of deterrence which the superpowers have relied on for the last 40 years -- the theory that neither side will attack the other with nuclear weapons for fear of devastating retaliation.

While this concept has worked, he said, "in recent years the evidence is very clear and undeniable that the stability of that concept is very much in question."

Specifically, the official said Soviet development of land-based, multiple nuclear-warhead mobile missiles meant that "the stability of offensive deterrence becomes much less clear." While the United States has lived with the "surreal" doctrine of offensive deterrence, he said, "evidence is mounting and undeniable it may not work in the years ahead."

The official said Reagan would not go so far as to demand a complete shift to defensive weapons. Rather, the official said Reagan will tell Gorbachev that the Soviets have upset the "underlying premises" of the offensive balance and suggest there is a "clear justification for examining if there is a better way" than reliance only on offensive weapons.

While the United States has already offered six versions of proposals to address what he called a substantial Soviet advantage in offensive missile capability, the official said the Soviets have made no serious response. Soviet sources have said that Foreign Minister Eduard Shevardnadze is to bring a proposal from Moscow when he meets with Reagan next week.

The official said Reagan had rejected the idea of a trade-off involving his missile defense program and Soviet offensive missiles because he "never had the misguided notion" that such a trade would improve the nation's security.

Gorbachev has indicated an interest in such a trade-off, the official said, because the Soviets are 10 years ahead of the United States in strategic defense and want to halt parallel U.S. efforts.

"It's a matter of them trying to whittle what they can away from ours," the official said. Both nations have reached a "joint conclusion" that they believe in strategic defense but the Soviets "want to circumscribe ours," he said.

On the ABM treaty, the official said the current level of research on Reagan's proposed missile defense program could continue for several years with existing treaty restraints, as Reagan has pledged to do.

He said the United States believes certain testing of the "Star Wars" program is permitted under the treaty, a position hotly challenged by the Soviets and by some American specialists. The ABM treaty bans tests of any space-based or air-based antimissile system or component of a system, and precludes the introduction of exotic new technology without new negotiations.

The official said the Soviets have expressed interest in revising some aspects of the 1972 treaty. Another official said later yesterday this interest by Moscow was clearly designed to limit U.S. testing of the Reagan missile defense program, not to permit testing of it.

While Americans may be alarmed at the idea of breaking the ABM treaty, the briefer said, the United States "simply may not be able to deal adequately with its defense" given the scale of the Soviet buildup. When combined with what the United States believes to be Soviet "violations of that [ABM] treaty," its value is "very much in question."