Defense Secretary Caspar W. Weinberger yesterday called research on the Strategic Defense Initiative (SDI) "an absolute necessity" for U.S. security, insisting that the controversial "Star Wars" missile defense program not become a bargaining chip to achieve an arms control agreement with the Soviet Union.

"If we stop our work on strategic defense and give it away at the negotiating table, we will forever lose one of history's best chances to end the shadow and fear of nuclear weapons," he said. Weinberger's remarks, in a speech to the Philadelphia World Affairs Council, mark the most comprehensive defense of SDI by an administration official since President Reagan was presented a Soviet proposal last week linking deep cuts in the superpowers' nuclear arsenals with cessation of work on SDI.

In what his aides billed as a major speech, Weinberger strongly reasserted the administration's commitment to SDI and cited recent Air Force success in enhancing the precision of low-power laser targeting against a rocket in space.

He sharply attacked SDI critics who "put blinders on strategy" and belong to the "blame-America-first school" in their analysis of the arms race. He said hopes of detente inspired by the Antiballistic Missile and SALT I arms control treaties of 1972 were "based on illusion" -- dashed by Soviet violations and a "massive buildup."

Much of Weinberger's speech was devoted to a criticism of the strategy of deterrence based on mutual assured destruction in which both superpowers have enough nuclear weapons to survive a first strike and punish the aggressor.

With Moscow's military modernization efforts bent on achieving first-strike capability, he said, a change in U.S. strategy "stands alone as the right, and indeed, the only thing to do -- to remove the threat of mutual destruction, regardless of Soviet activities."

Weinberger said opening a nuclear umbrella to shield U.S. weapons and citizens from a Soviet attack is the only "prudent course of action."

The administration, he said, is "determined to continue vigorous research until we know which defensive concepts are feasible . . . a research program into all forms of strategic defense is an absolute necessity for the long-range peace and security of America and its allies."