The Senate Armed Services Committee chopped $19.2 billion from President Reagan's fiscal 1987 defense spending authorization request yesterday, with the biggest cut coming in the Strategic Defense Initiative research program, also known as "Star Wars."

The $301 billion measure approved by the panel included $3.95 billion for SDI, $1.45 billion below what Reagan sought. This amount may turn out to be the highest possible for SDI, given that a majority of the committee's members are considered strongly pro-Pentagon and a House Armed Services subcommittee has approved even less for SDI, an amount $1.7 billion below the Pentagon's request.

Presidential spokesman Larry Speakes said such reductions would "undermine our entire response" to the Soviet missile buildup and make it "unlikely that we'll be able to make our timetable," which calls for a decision by the early 1990s on whether to develop SDI.

Sens. Sam Nunn (D-Ga.) and William S. Cohen (R-Maine), cosponsors of the Senate SDI reduction amendment, said Reagan's program is in trouble because it is "vague" and "confusing." Nunn said the large commitment of funds to SDI is detracting from technological research in other defense areas and that confused descriptions of SDI's goals are leading Congress to make deep cuts in the program.

Cohen and Nunn said the Senate committee will work out a "concept" of SDI that, if approved, could force some restructuring of research on the program.

One element advocated by Nunn and Cohen has been adopted. The committee has directed that some $487 million being cut from SDI be applied to other defense research programs. In addition, the two senators want researchers to examine short-term defenses that could meet an immediate threat to U.S. missiles, and put less emphasis on more futuristic, longer-term systems, such as a complete defense for cities.

Forty-eight senators have signed a letter calling for Star Wars funding for fiscal 1987 to be held at $3.2 billion, and the four members who promoted the letter -- William Proxmire (D-Wis.), J. Bennett Johnston (D-La.), Daniel J. Evans (R-Wash.) and Lawton Chiles (D-Fla.) -- are expected to meet next week to determine strategy for a floor amendment to reduce the SDI budget further.

Cohen said that without a concept for SDI, the committee's $3.95 billion allocation could be trimmed even more.

The committee-approved bill also sets the stage for another battle over development of a new antisatellite weapon. Congress last year barred testing of such a weapon against a target in space until the president certified that the Soviets have conducted such a test. The administration wants to proceed with the weapon, but critics say it could lead to a new arms race in space.

The Senate panel not only approved $28.5 million that the Pentagon sought to begin procurement of the new antisatellite missile and the full $278 million sought for research; it also adopted a provision that would end the moratorium against tests in space.

In contrast, House Armed Services subcommittees this week cut antisatellite research funds in half and eliminated all procurement money.

In another development, the Senate committee approved a measure that would delay the administration's plans to create a new institute, modeled after the Rand Corp., to support SDI research. The amendment, proposed by Sen. John W. Warner (R-Va.), would require the Defense Department and the General Accounting Office to report on the need for a new institute and would give Congress 30 days to consider the reports before the Pentagon could act.

Sen. Carl Levin (D-Mich.), who has expressed concern that the institute would be biased in favor of SDI, offered an amendment that would have barred creation of an institute until Congress authorized it. The Warner substitute was adopted along party lines, 10 to 9, but Levin and other senators -- including Proxmire and Paul Simon (D-Ill.) -- are expected to fight the institute on the Senate floor.