The Senate approved a top-to-bottom restructuring of the Strategic Defense Initiative (SDI) yesterday that would slow plans for early deployment of tiny rockets in space and shift emphasis to research on less exotic and longer-term defenses against missile attacks.

Brushing aside strong objections from the Bush administration, the Senate included the SDI overhaul as part of its $289 billion defense authorization bill for fiscal 1991 and then passed the bill by a vote of 79 to 16 before joining the House in taking off on a month-long summer recess.

In the latest in a series of congressional blows to the "Star Wars" program begun by President Ronald Reagan in 1983, the Senate voted 54 to 44 to stop issuing what critics called "blank checks" for the program and begin earmarking funds to reflect the priorities of Congress.

Foremost among these priorities was a freeze in spending for the "Brilliant Pebbles" plan being promoted by the Bush administration to orbit thousands of small rockets that would seek out and destroy a limited number of Soviet missiles before they hit U.S. targets.

The Senate reduced the administration's request for Brilliant Pebbles from $329 million to $129 million, its current funding level, and shifted the leftover money to research on ground-based defenses that would not breach U.S.-Soviet treaties and to other programs that would take longer to develop.

Congress has consistently reduced both Reagan's and President Bush's spending requests for SDI and appears certain to do so again this year. The Senate cut Bush's $4.7 billion request for SDI next year to $3.7 billion, and the bill that the House will consider next month would cut it to $2.9 billion. Current funding for SDI is about $3.8 billion.

But never before has either house attempted to rewrite the whole program. Yesterday's action was seen as especially significant because the move to restructure the program came from the Senate, which has supported SDI far more strongly than the House.

Moreover, it was proposed by moderate-to-conservative Sens. Jeff Bingaman (D-N.M.) and Richard C. Shelby (D-Ala.) and backed by Sen. Sam Nunn (D-Ga.), the powerful chairman of the Senate Armed Services Committee, who complained that the Pentagon has tried four different concepts for the program in three years, all without success. "No one can tell you where the money's coming from and yet they charge straight ahead as if it's all of a sudden going to appear in some kind of miracle," he said.

But SDI proponents, referring to new hostilities in the Persian Gulf, said the program is essential for defense against the proliferation of nuclear and other weapons in the Third World even as Cold War hostilities die down. Sen. Malcolm Wallop (R-Wyo.) also noted pointedly that work on projects that would benefit from the restructuring is "not coincidentally" being done in New Mexico and Alabama, home states of Bingaman and Shelby.

The only good news for Bush in yesterday's SDI showdown was that efforts to cut Star Wars spending below $3.7 billion were defeated. A move to cut another $600 million was rejected, 56 to 31, and a proposal to shift $400 million from defense to treatment of crack-afflicted babies was defeated, 54 to 43.

In a letter to Republican senators two days ago, Bush warned against the threatened SDI overhaul as well as further spending cutbacks. "In many ways, this {the overhaul} is more serious than a funding cut," he said. "As Congress imposes cuts in the defense budget, at the very least I need flexibility to manage our programs."

Bush did not threaten a veto if the overhaul was approved, but a statement of administration policy issued the same day said Defense Secretary Richard B. Cheney would recommend a veto if the bill did not include "proper funding or flexibility."

As a whole, the Senate's defense bill occupies the middle ground in the fierce fight that is expected this fall between the administration and congressional liberals over how far to go in dismantling the huge defense buildup of the 1980s in the face of dramatically reduced East-West tensions.

The Senate bill would cut $18 billion from the $306.9 billion that Bush requested last January, while the legislation that the House will consider in September cuts $24 billion.

As Bush requested, the Senate authorized two more B-2 "stealth" bombers so long as the plane successfully passes a series of performance tests. The House bill would end production of the radar-evading strategic bombers after completion of the 15 already authorized, far short of the 75 that the administration wants.

The Senate would cut 100,000 from the 2 million-member U.S. armed forces, nearly three times the size of the cut proposed by the administration but 29,500 less than the House measure would cut. The Senate would cut $1.3 billion for production of the rail-based MX missile but continue research and development for both the mobile MX and truck-based, single-warhead Midgetman missile. The House bill would kill both programs.

The Senate would also kill the $1 billion Milstar satellite program, designed to provide global communications in nuclear war, and the Army's new air-defense missile system. The Senate bill also would slow several other major programs, including the Army's light helicopter and Air Force's Advanced Tactical Fighter, for greater testing. Both houses refused to kill the Marine Corps' V-22 Osprey tilt-rotor aircraft, which the administration has been trying to scuttle for two years.