The U.S.-made Stinger antiaircraft missile is having an enormous impact, directly and indirectly, on the style of warfare in Afghanistan this summer. It has forced the Soviet Union to adopt new air and ground tactics against the American-backed insurgency and enabled the rebels for the first time to withstand direct Soviet-led assaults on their mountain strongholds.

Introduced into the 8-year-old war less than a year ago, the 35-pound, shoulder-fired weapon has led administration officials to revise radically their assessment of the badly fragmented, ragtag resistance's battlefield prospects as well as the missile's suitability to Third World guerrilla warfare.

"The battlefield equation has changed in favor of the resistance because the communist side is no longer able to employ its aircraft in tactically advantageous ways," said a senior administration official at a recent briefing.

The Stinger also has been sent to U.S.-backed insurgents fighting the Soviet-backed Marxist government in Angola, and the administration is assessing whether to provide Stingers to the contras in Nicaragua.

Once regarded as far too sophisticated technically for illiterate Afghan rebels to handle and too provocative politically to the Soviets to introduce in the Afghan war, the Stinger is now being hailed as the weapon that is turning the tables on the Soviets in this summer's round of fighting -- the heaviest of the war.

Such upbeat administration estimates of the rebels' prospects are not new and in the past have been followed by Soviet escalations in manpower or equipment, and changed tactics, that have offset the insurgency's gains.

For years, the Reagan administration hesitated to send any American-made arms, let alone the highly sophisticated Stinger, to the insurgents for fear of compromising its doctrine of "plausible deniability" regarding U.S. covert operations. Changing this policy became the cause celebre of conservatives in Congress and several small lobby groups supporting the Afghan rebel cause.

Having bowed early last year to this pressure, the administration seems to have overcome all its fears of the possible adverse consequences to sending advanced American and foreign weapons to the resistance.

The insurgents, a spokesman said, have been able to "neutralize" Soviet airpower, making "a big difference" in the ability of the guerrillas "to stand and fight." Their morale, after blunting two major Soviet-Afghan offensives last month, is higher than ever, he said.

Unwilling to discuss publicly U.S. covert aid, the official cited news reports crediting "Stingers and as well as other surface-to-air missiles" as a major factor in the battlefield turnaround.

U.S. and Soviet sources say the "other" antiaircraft missiles include British-made Blowpipes and Chinese ground-to-air missiles. The insurgents also are using their old Soviet-made Dashaka 12.7mm heavy machine guns with new effectiveness on low-flying Soviet and Afghan government aircraft.

The official said he was not suggesting the Afghan rebels, the mujaheddin, now have the power to inflict a military defeat on the estimated 115,000 to 120,000 Soviet troops in Afghanistan. "A possibility of a resistance victory . . . there never has been," he said.

But the rebels' stiffened resistance on the battlefield, he said, may help force the Soviets to make concessions they earlier rejected for a political settlement.

"If we . . . are correct about the current trends in the military struggle," he said, "that improves the chance that the Soviets will, sooner rather than later, come to a judgment that they're going to have to negotiate their way out of Afghanistan."

The Stinger's success is also having noticeable impact on supporters of the rebels in and outside Congress, who now are lobbying for more Stingers and other weapons, like long-range mortars and artillery to reach over Soviet minefields and knock out Soviet airfields, power stations and other important facilities.

"The Stinger has been highly effective. It has been a very, very successful program, far more than anyone expected," remarked Sen. Gordon J. Humphrey (R-N.H.), who pressed the administration last year to send the missile and is now urging it to send other arms.

"You need more Stingers than programmed now," said Rep. Charles Wilson (D-Tex.), a member of the House Permanent Select Committee on Intelligence and a leading congressional advocate of the Afghan rebel cause. "I think 2,000 is a pretty good number."

Sending such a large number of Stingers, congressional supporters of the insurgency admit, may require that Congress and the administration approve a supplemental appropriation for the program of U.S. covert military aid going to the Afghan rebels, now costing about $600 million a year.

The administration reportedly has already approved 600 Stingers this year for the seven guerrilla factions making up the U.S.-backed Afghan Alliance. However, one source said that for unexplained reasons, some have not been delivered, causing concern among several of the guerrilla groups.

U.S. personnel, mostly former Army antiaircraft specialists, have trained about 60 four-man guerrilla teams to fire the Stinger, whose five-mile range and infrared homing guidance system has so far proved largely invulnerable to any Soviet countermeasures.

Pakistani and U.S. intelligence sources estimate the Soviets and Afghan air forces are losing an average 1.3 to 1.4 aircraft per day. How many are being shot down by Stingers, Blowpipes, other antiaircraft missiles or heavy machine guns remains unclear.

David Isby, a director of the Committee for a Free Afghanistan, estimated that in view of the U.S. experience in Vietnam, it may be that one-half of all Soviet aircraft losses are the result simply of accidents.

U.S. officials, outside analysts and guerrilla spokesmen have given estimates of the ratio of targets hit to Stingers fired, varying between 4-to-10 and 7-to-10. All these estimates rely heavily on the rebels' battlefield reports, making it difficult to accurately determine a ratio.

Whatever the average, Soviet officials and media constantly cite the Stinger as evidence that the Reagan administration is escalating the war just when they say they are seriously seeking peace.

Encouraged by the missile's effectiveness, Andrew Eiva, whose Federation for American Afghan Action was a leading Stinger lobbyist last year, suggests that a British-made 81mm mortar, with a range of six kilometers, could make the eight main Soviet airfields in Afghanistan "untenable."

Mortars, Eiva said, proved highly effective in destroying U.S. aircraft on the ground during the Vietnam war and could "shut down every Soviet air base in Afghanistan."

Wilson suggested a longer-range mortar, reaching targets 12 to 13 kilometers away, would be even better because Soviet airfields and military camps are usually protected by mines and other defenses for a distance of six to eight kilometers. He also said the insurgents, whom he visited inside Afghanistan last February, also need equipment to pass through these targets' minefields.

The new Soviet tactics and mujaheddin counter-tactics emerged in late May during Soviet-Afghan offensives in eastern Paktia Province bordering Pakistan and in the Arghandad Valley in southern Afghanistan.

Soviet and Afghan warplanes now drop bombs from an altitude of 10,000 feet, rather than 2,000 to 4,000 feet, greatly reducing their accuracy, U.S. analysts say.

There also has been "a clearly reduced role," as the senior U.S. official put it, for the dreaded Soviet-made Hind Mi24 and Mi25 helicopter gunships that the Soviets and Afghan government troops depended on heavily for close-in support during offensives.

When used, the gunships now come in at extremely low altitudes to avoid surface-to-air missiles, and thus are vulnerable to the rebels' 12.7mm heavy machine guns. The number of aircraft being brought down in this manner has now risen, the senior official said.

The combination of high- and low-altitude antiaircraft weapons gives the insurgents an air defense system they never had before, enabling them to stand and fight rather than disperse when faced with a Soviet-Afghan attack.

The Soviets, to compensate for the loss of support aircraft, are using more artillery to try to blast the rebels from their positions. The senior U.S. official cited "a very substantial upgrade" in Soviet artillery and "a growing pattern" of "heavy reliance on artillery" over the past year to 18 months.

The Soviets have also sought to offset the insurgents' new air defense capabilites by committing their elite special forces, known as Spetznaz, to lead the assaults on their positions.

"We're seeing the Soviets now getting out of their APCs {armored personnel carriers} and fighting more directly in small-unit operations," the senior offical said. "That's of course a characteristic feature of the Spetznaz units. But it's true of other Soviet units as well that they are willing to get out from behind their armor and engage more directly in the battle."

In the May Soviet-led offensive in Paktia Province, the Soviets committed 5,500 troops, including at least one Spetznaz battalion, plus 2,000 to 3,000 Afghan government troops to dislodge an estimated 800 to 1,000 insurgents besieging an Afghan government garrison at Baran Khel.

The insurgents not only engaged in hand-to-hand fighting with the Soviets -- a rare occurrence in the war -- but forced them to retreat. The rebels have now resumed their siege of the garrison.

The Soviets are now taking heavier casualties, and the senior official predicted this would be "an increasing trend in the future."