EIGHT years after the "star wars" program was launched by President Reagan the United States is taking a big step toward actual deployment of a limited defense against ballistic missiles.The system would be designed to deal only with isolated missile launches - a far cry from the perfect astrodome shield originally envisioned by Strategic Defense Initiative (SDI) proponents. But, if erected, it would mark a major shift in US nuclear weapons policy, which has long shunned defenses and emphasized the superpower standoff of mutual assured destruction. The step is contained in the 1992 defense bill now nearing final approval in Congress. Senate and House negotiators have agreed that the legislation will explicitly call for deployment of a missile defense system that is consistent with the 1972 Anti-Ballistic Missile Treaty. The House in particular has long been a hotbed of SDI opposition. But as the program has been scaled back, shifting from visions of space lasers to ground-based rocket interceptors, House opposition has been muted. Now the question isn't whether to build defenses, but how. "There's a consensus now on defenses" in Congress that has developed without fanfare, says a key congressional aide. In order to reach this consensus, a shift in the pecking order of perceived threats to US national security was necessary. The possibility of an all-out, surprise nuclear attack from the Soviet Union, once at the top of the threat list, is now down much closer to the bottom. Its place has been taken by so-called "limited strike" scenarios, such as an accidental or rogue launch of a Soviet intercontinental ballistic missile (ICBM) or a terror strike by a third-world nation with ballistic-missile technology. As shown by the success of the Patriot missile in Saudi Arabia, the US already has technology that, if refined, is capab le of dealing with the threat of a limited strike. In contrast, defenses against a large Soviet attack would have necessarily included a technology much more difficult to perfect - some type of space weapon able to hit rocket boosters on the rise. President Bush never has seemed as committed to the star-wars program as was his predecessor. In the light of changing threats to the US, Mr. Bush last January officially refocused the SDI program away from comprehensive defense toward what he termed "G-PALS Global Protection Against Limited Strikes. Shortly thereafter, US Sen. Sam Nunn (D) of Georgia - a G-PALS enthusiast - won Senate approval for a 1992 defense bill provision that would mandate deployment of limited defenses, consistent with the ABM pact, by 1996. The House didn't have such a provision in its version of the bill. But in recent days House negotiators have agreed that the call for G-PALS should be part of the final legislation. It is not yet settled whether the 1996 target date will remain, or whether a more vague "when technology permits" standard will be used. Of such seeming legislative technicalities are major policy changes made. No defense program ever reaches a point after which deployment is irreversible - as the continuing B-2 controversy shows. But the House-Senate agreement on the defense bill removes the last major political roadblock to the beginnings of strategic defense. "This moves us far down the road toward deployment of ground-based defenses and perhaps beyond," notes John Isaacs, a Congress-watcher for the Council for a Livable World. Some important aspects of limited defenses aren't yet settled. One is what to do if the Soviets don't want to renegotiate the ABM Treaty. Under the terms of the the ABM pact, the US is allowed one antimissile site with 100 interceptor rockets. Since this is almost certain to be built at an old Safeguard ABM base in North Dakota, an ABM-compatible deployment would leave US coastal regions undefended. President Bush has called on the Soviets to renegotiate ABM Treaty terms, and Congress will echo this call in the defense bill. In the past, the Soviets have firmly rebuffed all such overtures. Lately, they have been sounding different, though with all the changes now roiling Soviet society it is hard to know who is speaking with authority. For instance: at a recent seminar in Washington, Maj. Gen. Viktor Samoilov, a member of the Russian Federation's State Committee on Defense, spoke quite positively about the prospect of some sort of mutual superpower deployment of a strategic shield. And space-based weapons are still a live issue. The Bush administration and, to a lesser extent, the Senate continue to be boosters of space-based interceptors better known as "Brilliant Pebbles." The House is far less enthusiastic about space-based weaponry and, though Brilliant Pebbles research will continue into 1992, it could spark a bitter fight in the 1993 budget.