The popular revolt against the Soviet invasion of Afghanistan has spread so widely that any prospect of a political solution appears doomed, and Moscow may find it impossible to withdraw its 80,000 troops, many diplomats here believe.

Not only would the Babrak Karmal government installed by the Soviets last December collapse for lack of popular support, but complete chaos would also grip the country.

Most diplomats think the Soviet occupation will last 5 to 10 or more years, and some predict Moscow shortly will have to increase its troop strength here to 400,000 -- almost as many as the number of U.S. military personnel in Vietnam during the peak of fighting there.

Some even expect cross-border attacks on Pakistan, where some rebel groups have their headquarters and which Moscow insists is funneling arms to the insurgent forces as well as providing sanctuaries for them.

If the diplomats here are correct in their assessment, which in some cases goes against the official positions of their government, even a U.S.-Soviet agreement, with the wide guarantees of noninterference in Afghan affairs that Moscow is demanding, would be thwarted by rebellious tribesmen emboldened by the Red Army's failure to crush them.

"If you put the clock back six or seven months, it might have been possible to work out a political solution which ignored the rebel forces. I don't believe that is possible now," said a Western European diplomat who is considered one of the most astute analysts here.

Asian and other nonaligned diplomats interviewed during nearly a week's stay in the Afghan capital agreed with his pessimistic conclusions.

One diplomat put forth what he called "a cynical scenario," namely that "apathy and acceptance will set in over time when the Afghans realize there is nothing they can do."

However, there is no evidence that this attitude of surrender has yet appeared. "The rebels keep getting clobbered, and yet they keep coming back. I am constantly amazed at these guys' ability to come back for more," said one Western envoy.

The Soviets quite clearly were stunned by the depth of resistance they encountered when they moved their troops into Afghanistan last December.

They seemed to believe that their massive display of force would cow the Afghans into submission, and they did very little fighting last January and February.

"That was totally unwise. Surely they must have known that in the long run this rebel movement would build up, be more effective. They should have known that. After all, Afghanistan is a neighboring country," said a European diplomat.

Another diplomat felt the Soviets looked at Afghanistan through European eyes, and expected to meet the same capitulation to a show of force that they got in Hungary and Czechoslovakia. But the Afghans, fiercely independent and with a long tradition of repelling invaders, refused to roll over and play dead.

"This naivete is perhaps an element of the continuing problems the Russians are having in Afghanistan. Many Afghans, unlike East Europeans, do not realize when their position is hopeless," he reported back to his government.

The rebel forces have gained strength and, more important, popular support since the Soviet invasion. "The presence of Soviet troops has polarized the population and unified the opposition," said one observer here. "But tribal and family differences have not been buried."

Therefore, it appears likely the rebels would begin fighting among themselves if the unifying factor of the Soviet troop presence in their country were ended.

"If there was a withdrawal," said a Western diplomat, "this place would deteriorate into chaos, and I don't think that would be acceptable to the Soviets."

Indeed, many observers believe that the upheaval accompanying a Soviet withdrawal would be so intense that Soviet troops would literally have to battle their way out of the country. Such a spectacle would severely compromise the image of strength the Soviets like to project to their client states in Eastern Europe and to the largely Islamic republics of Soviet Central Asia adjacent to Afghanistan.

Even now there are reports that the revolt has spread to the previously peaceful Afghan provinces bordering the Soviet Union's Central Asian republics, where there have been great fears that the new Islamic militancy would catch hold and lead to anticommunist demonstrations.

The Soviet position here has been further undermined by the continued feuding between the two factions of the ruling Marxist People's Democratic Party of Afghanistan, which it tried to unify under Babrak. There are still nightly assassinations of members of one faction by the other, and Babrak himself is reported to sleep in different houses every night to lessen the chance that he will be killed.

There are also unconfirmed reports circulating here that members of the Khalq faction of the party, which supported deposed presidents Nur Mohammad Taraki and Hafizullah Amin and which is considered more independent and less pro-Moscow than Babrak's Parcham faction, have been cooperating with the rebels.

It is hard to know whether those reports are true.But it is significant that they are circulating among Afghans here.

The rebels have improved greatly as fighters, and they appear to have the sympathy of a wide swath of the population in this capital city, despite a continual barrage of propaganda on radio and television as well as in the newspapers attempting to portray them as anit-Islamic bandits.

But they have a lot to learn. Moreover, observers here believe their only chance of winning is to get massive aid from the West -- the kind of help the Soviets accuse them of getting but which it appears they are not.

Residents of Kabul, believed not to be part of the organized resistance, angrily accost Americans on the streets and demand to know why the United States is not giving help to the rebels.

But the only chance they have of getting large amounts of needed aid, and perhaps actual intervention by the West, is if the Soviets move from Afghanistan into the oil-rich Persian Gulf states of Iran and Saudi Arabia.

Many analysts here and in Washington believe the real reason the Soviets invaded Afghanistan was to position themselves for a strike at Iran's oil wealth, and that only unexpected ferocity of the Afghan rebels has stopped them.

The ferocity should be frightening to the Soviet invaders if they study their history. Afghanistan has never been kind to invaders.

In 1842, the British were forced by a growing popular revolt to pull their 16,500 troops and camp followers out of Kabul in the dead of winter. They lost all but a handful of these troops during the retreat, one of the greatest disasters in British military history, as tribesmen constantly attacked them from the hills along the way back to India.