Sen. Sam Nunn (D-Ga.), chairman of the Senate Armed Services Committee, said Tuesday evening that the United States and Soviet Union should consider developing a limited defense against accidentally launched ballistic missiles.

Nunn told the private Arms Control Association that such a system would be an "insurance policy" for the country and a sensible alternative to what he called President Reagan's flawed Strategic Defense Initiative (SDI).

Rep. Les Aspin (D-Wis.), chairman of the House Armed Services Committee, was reported yesterday by aides to be considering making a similar recommendation soon.

Under Nunn's plan, the SDI program would change its goal from that of a space-based shield against a massive Soviet attack to what he called "the reasonable goal" of stopping a single missile.

Nunn indicated that such a limited defense would consist of ground-based missile interceptors "spread throughout the country." He said "if properly designed," the defensive system would not be integrated with offensive nuclear forces to pose a first-strike threat to the Soviet Union.

The Soviets have complained that SDI might be used to protect the United States against Soviet nuclear retaliation for a U.S. first strike.

Nunn characterized the limited defense as "a logical follow-on" to the U.S.-Soviet agreement last year to establish Nuclear Risk Reduction Centers, which will exchange data on military forces to help avert international misunderstanding. Nunn was a supporter of the centers.

He added that "such defensive deployments might be possible within the terms" of the 1972 Antiballistic Missile (ABM) Treaty, or "at most, require a modest amendment." The ABM Treaty permits both sides to deploy up to 100 ground-based missile interceptors.

The Soviets have such interceptors deployed near Moscow, while the United States abandoned its system near missile silos in North Dakota in the 1970s.

The prospect of such endorsements from two of the principal military experts in the Democratic Party caused surprise and some anxiety among congressional critics of missile defenses yesterday.

The officials said it could pressure the six Democratic presidential candidates, each of whom has strongly criticized the SDI program, to support deployment of a more-modest missile defense.

The primary drawback of such a system is that its limited capability might not be believed by the Soviet Union, which would then build a more-capable system and start a new arms race, the officials said.