The Bush administration has ordered a scaling back of the aims of the 6-year-old Strategic Defense Initiative (SDI) effort to develop a defense against thousands of Soviet ballistic missiles, senior U.S. officials said yesterday.

The new aim will be to develop a defensive system capable of protecting the United States and its military forces against an attack by fewer than 100 missiles launched simultaneously by any potential adversary, including long-range Soviet missiles or shorter-range tactical missiles fired by countries such as Iraq, the officials said.

The decision to give the program a new focus primarily reflects lower budget projections and declining congressional support for the SDI program, as well as continuing scientific skepticism about the feasibility of blocking an attack by many Soviet missiles, the officials said. It also reflects diminished fear of a Soviet missile attack.

The administration hopes to boost the fortunes of its new version of SDI by linking it more closely to the successful interceptions of 33 Iraqi Scud missiles by Patriot antiaircraft batteries in the Persian Gulf War, the officials said on condition they not be identified.

The reduced aims contrast with a substantial increase in the administration's forthcoming budget for SDI and may still come under fire from some congressional and scientific critics.

President Bush announced the decision in his State of the Union address Tuesday night, citing what he called the "remarkable" success of the Patriot in defending Israeli and Saudi citizens against Iraqi attacks with modified Scud tactical missiles. He said that in the future, the SDI program would concentrate "on providing protection from limited ballistic missile strikes, whatever their source."

The overall research budget for a successor to the Patriot system would be more than doubled under the proposed SDI budget to be released this weekend, the officials said. The budget, estimated by one official at more than $4 billion, also would include vastly higher spending on space-based weapons systems that could not be deployed without violating the 1972 U.S.-Soviet Antiballistic Missile (ABM) treaty.

SDI managers now say they want to deploy space-based weapons in the late 1990s, after first developing new tactical missile defenses that could be transported anywhere U.S. forces are deployed overseas. While the former goal remains politically controversial, the latter appears to enjoy broad support.

Officials say that less than 5 percent of the estimated $23 billion spent by SDI since 1984 has been allocated for systems designed to blunt the type of tactical, or theater, missile threat posed by Iraqi military forces. Congress last year overrode the administration's objections and created a Theater Missile Defense Initiative funded at $390 million, roughly triple the amount proposed by Bush for such research.

SDI director Henry Cooper said last Saturday on PBS that "SDI has nothing to do with {the} Patriot {system deployed to the Middle East} in terms of the management of the program." An Army spokesman agreed, saying, "The Patriot systems fighting now in Israel and Saudi Arabia were developed, bought, and paid for by and for the U.S. Army. They are not some product of 'Star Wars' technology or 'Star Wars' funding."

But SDI supporters nonetheless have sought to link the Patriot missile success in the Middle East to SDI's overall goal of defending against a missile attack.

"The Patriot is a piece of technology that emerged from the SDI program, in the sense that -- well, some of the research that went into it was, and some of the concepts . . . emerged from that program," White House press secretary Marlin Fitzwater said yesterday. "So in that sense . . . its success does give us another degree of optimism about pursuing those kinds of technologies."

Sen. Arlen Specter (R-Pa.) said, "It shows it can work. It's a real shot in the arm for SDI."

Sen. Richard C. Shelby (D-Ala.), whose state has a major center of Army research on SDI, said Patriot had "captured people's imagination," and added, "It reminds me of Sputnik." He referred to the 1957 Soviet satellite that galvanized support for the multibillion-dollar U.S. space program.

Former defense secretary Harold Brown said recently however that linking the two programs "is just partisan nonsense." He said, "The 'Star Wars' program is based largely on the idea . . . of space-based weapons. In fact, space-based weapons would not work at all against Scud missiles."

Brown was referring to the Scud flight time of 7 to 8 minutes, which is so brief that it precludes interception by a weapon requiring much longer to traverse the necessary distance from space.

While Patriot missiles have downed each of the 33 modified Scud missiles they attempted to hit since the beginning of the war with Iraq two weeks ago, SDI research weapons launched from the ground have only twice hit mock ballistic missile warheads in space. The most recent success occurred on Monday, when a ground-based missile fired from Kwajalein Atoll in the Pacific struck a warhead launched from Vandenburg Air Force Base in California.

The intercepting missile in that experiment was not designed to be deployed in a defensive system, SDI officials say. Years of additional research will be needed to ready its successor for potential use, along with "Brilliant Pebbles" space-based weapons, under a system termed "Global Protection Against Limited Strikes" or GPALS by its supporters and "SDI-Lite" by its critics.

The estimated cost of developing the system is roughly $40 billion, one official said.

"This is a scaled-back mission that can actually be done, which replaces a mission that was always a technological will-o'-the-wisp," said Matthew Bunn, a space weapons expert at the private Arms Control Association here.

"The first thing to notice is how far we have traveled from President Reagan's vision . . . to render nuclear weapons impotent and obsolete," said Albert Carnesale, an engineer and professor of public policy at Harvard University. He added, however, that the deployment of thousands of weapons in space and on Earth "to protect my forces in the field is probably not cost effective" and would require forgoing military benefits from the ABM treaty.