The head of the Strategic Defense Initiative said here today that scientific opposition to his program is withering away and that "only a few diehards" continue to question the feasibility of developing a defensive shield against Soviet missile attack.

The remarks, made by Air Force Lt. Gen. James A. Abrahamson at the second annual Space Symposium, contrast with the views of SDI critics in the scientific community who say opposition to "Star Wars," as the program is popularly known, is growing.

One measure of the opposition, the critics say, is an anti-SDI petition drive that has so far garnered the signatures of more than 2,100 science professors and 1,700 science graduate students at about 50 campuses and other research centers. The numbers have grown by several hundred in recent weeks.

Petition signers declare their opposition and refusal to work on SDI-related projects. The signers include 16 Nobel laureates and 58 percent of the physics professors at the 14 most prestigious physics departments.

Abrahamson characterized the petitions as "plaintive letters" from students who do not represent mainstream views.

"There are only a few diehards that are still saying this just doesn't make sense," Abrahamson told several hundred military and commercial enthusiasts of space exploitation. The symposium is sponsored by the United States Space Foundation, a private group that encourages space exploitation.

Abrahamson said the SDI had already achieved so many successes in its research on antimissile defenses that "the question is no longer if we can do something. The question is how fast."

Ordinarily SDI officials emphasize that their program is one of research and that it will be years before a feasibility decision can be made.

Abrahamson also said that congressional budget cuts have forced the SDI to scale back some of its programs.